Ubuntu – Dual booting Windows 7 and Ubuntu 19.04

I'm not a beginner with creating a dual booting system, but I guess I have a lot to learn still because I have been struggling with the current system for days.

I started off with a Windows 7 system that occupied two physical disks. I took the second and made it into an Ubuntu system. Currently, it looks like this:

/dev/sda1     NTFS    (labeled as "system reserved")
/dev/sda2     NTFS    Windows 7
/dev/sda3     NTFS    (Data for Windows 7 system to access)

/dev/sdb1     FAT32   Mounted on /boot/efi (flags "boot" and "esp")
/dev/sdb3     Ext4    Mounted on /

(There was a /dev/sdb2, but I deleted it and merged it into
/dev/sdb3 with gparted.)

I installed Ubuntu 19.04 and it is working fine. But I cannot boot into Windows 7. I want to add Windows 7 to the grub menu or at least access it; I can't even do that now.

So, I've tried the following:

• Run sudo update-grub.
• Run sudo os-prober after mounting /dev/sda2.
• Running boot-repair on the computer and also with a bootable
  live disk.
• Running grub-customizer.

None of them work.

For boot-repair, I've tried several things such as "Reinstall GRUB" and
"Restore MBR". If I do the "Recommended repair", I get a "GPT detected. Please
create a BIOS-Boot partition (>1 MB, unformatted filesystem, …).
…Alternatively, you can retry after activating the [Separate
/boot/efi partition:] option." After running boot-repair for many
years on other systems, this is the first time I've seen this message.

With grub-customizer, it created these lines for /dev/sda2
(Chainloader):

set root='(hd0,2)'
search --no-floppy --fs-uuid --set XXX
drivemap -s (hd0) ${root}
chainloader +1

When I reboot, I get errors like "no such device: XXX", "can't find command drivemap", and "disk hd0,2 not found".

Of all the times I've gone through this problem with other computers, I guess this is the oldest I've had. (It has Windows 7, after all.) Likewise, the BIOS is old as well. So, that is stumping me. I'm also a bit concerned I've some how destroyed the (Windows) system. I tried the "Restore MBR" option of boot-repair and wrote the MBR to /dev/sda2 and I still couldn't boot into Windows — it goes to grub instead. (When I realised this, then I thought I need to ask others for help…this feels bad now.)

The Ubuntu side is a fresh install and if I need to somehow destroy that to undo a mistake I made in order to get it to dual boot, I'm fine with that.

Can anyone offer suggestions on what I should try? As I'm honestly guessing, I'm sure the answer is right in front of me but some suggestions on what I should try and perhaps the order would help.