Is that after you followed the steps I recommended above?
Not yet. Taking photos over those almost 4,400 lines took a lot of time.
First of all, that error typically isn’t related to the bootloader. Chances are, the bootloader was working fine. But now you have all kinds of things going on there.
When I took above photos a short while ago, EOS failed one the very same line as before. The GRUB menu looked identical, too. At least visually nothing seemed to have changed to that point. (Of course I cannot speak about anything beyond that point of failure.)
Refind is installed,
rEFInd was installed before. Because my one-year long experience with a number of different Linux distributions is: GRUB and other Linux bootloader-related things are very prone to failures. I stumbled upon rEFInd and decided to – from then on – precautionally install it on every Linux machine.
grub configs were changed(maybe), EFI settings changed, etc, etc. Now we probably have to mess around with the bootloader just to get things back to a sane config to start troubleshooting.
Well, on Saturday, 10:36 am, I created a copy of esp partition sda1 via a partition manager. It is a .img file. So even if sda1 is damaged, it could be replaced, correct?
addendum: There is also a copy of the not-booting EOS partition sda2, but not a .img file, rather a problematic replicated partition spanning over a whole 2 TB NVMe SSD without a partition table.