MacOS – No bootable device – insert boot disk and press any key

It's possible the drive is failing. Before doing anything else, backup your data. Holding the option key brings up the startup manager which shows any available volumes.

Do you have multiple drives connected to your computer? Maybe it is trying to boot using the wrong drive.

The value of 726562776 in the GPT might be wrong. My guess would be to change the value to 976695384. I would wait and see if someone else has a better idea.

The command to do this are given below. Here, I assume you have booted to a newer version of macOS Recovery.

gpt -f remove -i 2 disk0 
gpt -f add -i 2 -b 409640 -s 976695384 -t apfs disk0

The older versions of macOS Recovery would require the longer commands given below.

diskutil unmountDisk /dev/disk0
gpt remove -i 2 /dev/disk0 
diskutil unmountDisk /dev/disk0
gpt add -i 2 -b 409640 -s 976695384 -t 7C3457EF-0000-11AA-AA11-00306543ECAC /dev/disk0

Why 976695384?

• Reason 1: The error message from your question, which is repeated below.

  warning: nx_block_count is 122086923, while device block count is 90820347
  

  Since the device block size is 4096 bytes and the sector size is 512 bytes, the conversion requires multiplying by 8, as shown below.

  122086923 blocks * 8 sectors per block = 976695384 sectors
  

• Reason 2: This is the largest sector size that will hold complete blocks. If you take 976695387 sectors and convert to whole blocks by dividing by 8 and discarding the remainder of 3, you get the following.

  976695387 sectors / 8 blocks per sector = 122086923 blocks
  

  Next, convert back to sectors, as shown below.

  122086923 blocks * 8 sectors per block = 976695384 sectors
  

  This can be computed in your head. Basically, you divide the last three digits of 976695387 by 8 to get the remainder. In other words, 387 / 8 = 48 with remainder of 3. Next, subtract the remainder from 976695387 to get the result. In other words, 976695387 - 3 = 976695384. Why the last three digits? Well, 976695387 can be written as 976695 * 1000 + 387. Since 1000 is always evenly divisible by 8, you only need the last three digits to get the remainder.