How to Modify an Installation ISO and Keep It Bootable

I know there have been quite a few similar questions, but they aren't specific enough.

I have a Windows 10 x64 installation ISO and I am trying to extract the files make some modifications and then make a new ISO from the extracted/modified files.

It basically works just fine, but the problem is that I can't figure out how to make it UEFI bootable again (in legacy/BIOS mode it boots just fine).

I have gone through countless posts following instructions on how to do this, but none of them worked for my Windows 10 ISO. Most of these posts only mention older versions of Windows and those mentioning Windows 10 don't specify if they got it to work with a current x64 version and if they were able to UEFI boot from it. (Probably not, because is surely doesn't work for me.)

I ended up trying to replicate the output of isoinfo -d -i ./windows10.iso and dumpet -i ./windows10.iso as much as I could.

And this is as close as I was able to get:
(Edit: Updated with -eltorito-alt-boot as suggestes by telcoM)

Original ISO (isoinfo):

$ isoinfo -d -i ./original.iso
CD-ROM is in ISO 9660 format
System id: 
Volume id: CCCOMA_X64FRE_EN-US_DV9
Volume set id: CCCOMA_X64FRE_EN-US_DV9
Publisher id: MICROSOFT CORPORATION
Data preparer id: MICROSOFT CORPORATION, ONE MICROSOFT WAY, REDMOND WA 98052, (425) 882-8080
Application id: CDIMAGE 2.56 (01/01/2005 TM)
Copyright File id: 
Abstract File id: 
Bibliographic File id: 
Volume set size is: 1
Volume set sequence number is: 1
Logical block size is: 2048
Volume size is: 2411879
El Torito VD version 1 found, boot catalog is in sector 22
NO Joliet present
NO Rock Ridge present
Eltorito validation header:
    Hid 1
    Arch 0 (x86)
    ID 'Microsoft Corporation'
    Key 55 AA
    Eltorito defaultboot header:
        Bootid 88 (bootable)
        Boot media 0 (No Emulation Boot)
        Load segment 0
        Sys type 0
        Nsect 8
        Bootoff 202 514

Modified ISO (isoinfo):

$ isoinfo -d -i ./modified.iso
CD-ROM is in ISO 9660 format
System id: 
Volume id: CCCOMA_X64FRE_EN-US_DV9
Volume set id: CCCOMA_X64FRE_EN-US_DV9
Publisher id: Microsoft Corporation
Data preparer id: MICROSOFT CORPORATION, ONE MICROSOFT WAY, REDMOND WA 98052, (425) 882-8080
Application id: CDIMAGE 2.56 (01/01/2005 TM)
Copyright File id: 
Abstract File id: 
Bibliographic File id: 
Volume set size is: 1
Volume set sequence number is: 1
Logical block size is: 2048
Volume size is: 2411275
El Torito VD version 1 found, boot catalog is in sector 1506
NO Joliet present
NO Rock Ridge present
Eltorito validation header:
    Hid 1
    Arch 0 (x86)
    ID 'Microsoft Corporation'
    Key 55 AA
    Eltorito defaultboot header:
        Bootid 88 (bootable)
        Boot media 0 (No Emulation Boot)
        Load segment 0
        Sys type 0
        Nsect 8
        Bootoff 8CD 2253

Difference between the above isoinfo outputs:

$ diff <(isoinfo -i ./original.iso) <(isoinfo -i ./modified.iso)
5c5
< Publisher id: MICROSOFT CORPORATION
---
> Publisher id: Microsoft Corporation
14,15c14,15
< Volume size is: 2411879
< El Torito VD version 1 found, boot catalog is in sector 22
---
> Volume size is: 2411275
> El Torito VD version 1 found, boot catalog is in sector 1506
29c29
<         Bootoff 202 514
---
>         Bootoff 8CD 2253

Original ISO (dumpet):

$ dumpet -i ./original.iso
Validation Entry:
        Header Indicator: 0x01 (Validation Entry)
        PlatformId: 0x00 (80x86)
        ID: "Microsoft Corporation"
        Checksum: 0x494c
        Key bytes: 0x55aa
Boot Catalog Default Entry:
        Entry is bootable
        Boot Media emulation type: no emulation
        Media load segment: 0x0 (0000:7c00)
        System type: 0 (0x00)
        Load Sectors: 8 (0x0008)
        Load LBA: 514 (0x00000202)
Section Header Entry:
        Header Indicator: 0x91 (Final Section Header Entry)
        PlatformId: 0xef (EFI)
        Section Entries: 1
        ID: ""
Boot Catalog Section Entry:
        Entry is bootable
        Boot Media emulation type: no emulation
        Media load address: 0 (0x0000)
        System type: 0 (0x00)
        Load Sectors: 1 (0x0001)
        Load LBA: 516 (0x00000204)

Modified ISO (dumpet):

$ dumpet -i ./modified.iso 
Validation Entry:
        Header Indicator: 0x01 (Validation Entry)
        PlatformId: 0x00 (80x86)
        ID: "Microsoft Corporation"
        Checksum: 0x494c
        Key bytes: 0x55aa
Boot Catalog Default Entry:
        Entry is bootable
        Boot Media emulation type: no emulation
        Media load segment: 0x0 (0000:7c00)
        System type: 0 (0x00)
        Load Sectors: 8 (0x0008)
        Load LBA: 2253 (0x000008cd)
Section Header Entry:
        Header Indicator: 0x91 (Final Section Header Entry)
        PlatformId: 0xef (EFI)
        Section Entries: 1
        ID: ""
Boot Catalog Section Entry:
        Entry is bootable
        Boot Media emulation type: no emulation
        Media load address: 0 (0x0000)
        System type: 0 (0x00)
        Load Sectors: 2984 (0x0ba8)
        Load LBA: 1507 (0x000005e3)

Difference between the above dumpet outputs:

$ diff <(dumpet -i ./original.iso) <(dumpet -i ./modified.iso)
13c13
<       Load LBA: 514 (0x00000202)
---
>       Load LBA: 2253 (0x000008cd)
24,25c24,25
<       Load Sectors: 1 (0x0001)
<       Load LBA: 516 (0x00000204)
---
>       Load Sectors: 2984 (0x0ba8)
>       Load LBA: 1507 (0x000005e3)

I have written a script to completely reproduce the problem with the same ISO that I'm using:

#!/usr/bin/env bash

##################################
# Download the Windows 10 x64 ISO

WIN10_IMG_DESTINATION="./windows.iso"
WIN10_IMG_ARCH="x64"

if [ ! -f "${WIN10_IMG_DESTINATION}" ]; then
    if [[ "$WIN10_IMG_ARCH" == "x86" ]] || [[ "$WIN10_IMG_ARCH" == "i386" ]] ; then
        echo "Retrieving the x86 Windows 10 iso URL..."
        WINDOWS_10_ISO_URL=$(curl -LsI -o /dev/null -w %{url_effective} "https://windows101tricks.com/1903-iso-32")
    else
        echo "Retrieving the x64 Windows 10 iso URL..."
        WINDOWS_10_ISO_URL=$(curl -LsI -o /dev/null -w %{url_effective} "https://windows101tricks.com/1903-iso-64")
    fi

    echo "Making sure the URL comes from a trusted Microsoft (sub)domain..."
    if [[ $WINDOWS_10_ISO_URL == https://software-download.microsoft.com/* ]] ; then
        echo "Downloading the Windows 10 installation iso..."
        wget "$WINDOWS_10_ISO_URL" -O "$WIN10_IMG_DESTINATION"
    else
        echo "URL validation failed. Please download the Windows 10 iso manually."
        exit 1
    fi
else
    echo "Windows 10 iso already exists. Skipping download..."
fi
#
##################################



# Variable containing the path to the windows.iso
WIN10_IMG="$WIN10_IMG_DESTINATION"

TMP="./tmp"
ISO_FILES="${TMP}/iso-files"
ISO_MP="${TMP}/iso-mountpoint"

# Remove ./tmp if it already exists, then create ./tmp/iso-files and ./tmp/iso-mountpoint
rm -rf "${TMP}"
mkdir -p "${ISO_FILES}"
mkdir -p "${ISO_MP}"
# Extract the files fromt he ISO to ./tmp/iso-files
sudo mount -t udf "${WIN10_IMG}" "${ISO_MP}"
sudo cp -Rva ${ISO_MP}/* "${ISO_FILES}"
sudo umount "${ISO_MP}"

# Make modifications to the Windows ISO
#BOOT_DIR="${ISO_FILES}/efi/microsoft/boot"
#sudo mv "${BOOT_DIR}/cdboot.efi" "${BOOT_DIR}/tmp.efi"
#sudo mv "${BOOT_DIR}/cdboot_noprompt.efi" "${BOOT_DIR}/cdboot.efi"
#sudo mv "${BOOT_DIR}/tmp.efi" "${BOOT_DIR}/cdboot_noprompt.efi"

# Extract the boot.img (didn't help at all)
#BOOT_SECTOR_LENGTH="$(isoinfo -d -i "${WIN10_IMG}" | grep "Nsect " | grep -o "[^ ]*$")"
#STARTING_SECTOR="$(isoinfo -d -i ./vm-files/windows10.iso | grep "Bootoff " | grep -o "[^ ]*$")"
#dd if="${WIN10_IMG}" of="${ISO_FILES}/boot.img" bs=2048 count="${BOOT_SECTOR_LENGTH}" skip="${STARTING_SECTOR}"


# Extract boot load segment address and size
BOOT_LOAD_SEG="$(dumpet -i "${WIN10_IMG}" | grep "Media load segment: " | cut -d ':' -f2 | cut -d ' ' -f2)"
BOOT_LOAD_SIZE="$(dumpet -i "${WIN10_IMG}" | grep "Load Sectors: " | grep -o "[^:]*$" | cut -d ' ' -f2 | head -1)"

# Extract meta data :
SYSTEM_ID="$(isoinfo -d -i "${WIN10_IMG}" | grep "System id: " | cut -d ' ' -f3-)"
VOLUME_ID="$(isoinfo -d -i "${WIN10_IMG}" | grep "Volume id: " | cut -d ' ' -f3-)"
VOLUME_SET_ID="$(isoinfo -d -i "${WIN10_IMG}" | grep "Volume set id: " | cut -d ' ' -f4-)"
#PUBLISHER_ID="$(isoinfo -d -i "${WIN10_IMG}" | grep "Publisher id: " | cut -d ' ' -f3-)" # Always uppercase
PUBLISHER_ID="$(isoinfo -d -i "${WIN10_IMG}" | grep "ID '" | cut -d "'" -f2)"
DATA_PREPARER_ID="$(isoinfo -d -i "${WIN10_IMG}" | grep "Data preparer id: " | cut -d ' ' -f4-)"
APPLICATION_ID="$(isoinfo -d -i "${WIN10_IMG}" | grep "Application id: " | cut -d ' ' -f3-)"
COPYRIGHT_FILE_ID="$(isoinfo -d -i "${WIN10_IMG}" | grep "Copyright file id: " | cut -d ' ' -f4-)"
ABSTRACT_FILE_ID="$(isoinfo -d -i "${WIN10_IMG}" | grep "Abstract file id: " | cut -d ' ' -f4-)"
BIBLIOGRAPHIC_FILE_ID="$(isoinfo -d -i "${WIN10_IMG}" | grep "Bibliographic file id: " | cut -d ' ' -f4-)"

# Create a new ISO image using mkisofs
# (.mkisofsrc is necessary, because some options are not available on the cli directly)
rm ".mkisofsrc"
echo "APPI=${APPLICATION_ID}" >> ".mkisofsrc"
echo "COPY=${COPYRIGHT_FILE_ID}" >> ".mkisofsrc"
echo "ABST=${ABSTRACT_FILE_ID}" >> ".mkisofsrc"
echo "BIBL=${BIBLIOGRAPHIC_FILE_ID}" >> ".mkisofsrc"
echo "PREP=${DATA_PREPARER_ID}" >> ".mkisofsrc"
echo "PUBL=${PUBLISHER_ID}" >> ".mkisofsrc"
echo "SYSI=${SYSTEM_ID}" >> ".mkisofsrc"
echo "VOLI=${VOLUME_ID}" >> ".mkisofsrc"
echo "VOLS=${VOLUME_SET_ID}" >> ".mkisofsrc"

sudo rm "${WIN10_IMG}.tmp.iso"
sudo mkisofs \
  -no-emul-boot \
  -b boot/etfsboot.com \
  -boot-load-seg "${BOOT_LOAD_SEG}" \
  -boot-load-size "${BOOT_LOAD_SIZE}" \
  -eltorito-alt-boot \
  -e efi/boot/bootx64.efi \
  -no-emul-boot \
  -iso-level 2 \
  -boot-info-table \
  -udf \
  -D \
  -N \
  -relaxed-filenames \
  -allow-lowercase \
  -o "${WIN10_IMG}.tmp.iso" \
  "${ISO_FILES}"

rm ".mkisofsrc"


# Print the variables that we gathered
echo
echo "Extracted meta data (form original image):"
echo "BOOT_LOAD_SEG: ${BOOT_LOAD_SEG}"
echo "BOOT_LOAD_SIZE: ${BOOT_LOAD_SIZE}"
echo "-------"
echo "SYSTEM_ID: ${SYSTEM_ID}"
echo "VOLUME_ID: ${VOLUME_ID}"
echo "VOLUME_SET_ID: ${VOLUME_SET_ID}"
echo "PUBLISHER_ID: ${PUBLISHER_ID}"
echo "DATA_PREPARER_ID: ${DATA_PREPARER_ID}"
echo "APPLICATION_ID: ${APPLICATION_ID}"
echo "COPYRIGHT_FILE_ID: ${COPYRIGHT_FILE_ID}"
echo "ABSTRACT_FILE_ID: ${ABSTRACT_FILE_ID}"
echo "BIBLIOGRAPHIC_FILE_ID: ${BIBLIOGRAPHIC_FILE_ID}"

# Show difference between new and old image as reported by isoinfo
echo
echo "-------------- isoinfo diff -----------------"
diff <(isoinfo -d -i "${WIN10_IMG}") <(isoinfo -d -i "${WIN10_IMG}.tmp.iso")

# Show difference between new and old image as reported by dumpet
echo
echo " -------------- dumpet diff -----------------"
diff <(dumpet -i "${WIN10_IMG}") <(dumpet -i "${WIN10_IMG}.tmp.iso")


# Overwrite the original ISO with the new one
#sudo rm "${WIN10_IMG}"
#sudo mv "${WIN10_IMG}.tmp.iso" "${WIN10_IMG}"