Can you please tell me why a new offline installation of Endeavour on the same SSD would boot normally before the update?
The offline installer has an older kernel and the packages are not up todate. So when you update it that is where the issue happens. The linux firmware issue is one that has to have manual intervention to fix that so you can update.
From there I cannot say what the issue is. It also looks like you are able to boot into MX linux? I am not convinced that you cannot use the endeavourOS live iso and boot on it and arch-chroot into the installed system then reinstall grub and update grub on eos. Then see if it boots properly on eos where it controls the grub bootloader. You haven’t said whether you have tried this but i assume it’s because you have said arch-chroot hasn’t worked for you.
Edit: I also see that your Bios is 2014. Have you checked to see whether there is a newer Bios update available for this laptop.
There have been some bios updates if you look at this page:
Update: I reinstalled EndeavourOS with all three kernels: regular, LTS, and ZEN. I used the online installer. Results: 1) The regular kernel has the known issues and won’t boot. 2) The LTS kernel is the only one that boots without issues!!! 3) The ZEN kernel was able to boot once, but there was a stop job issue on startup and again on shutdown. It took a very long time. After that, I couldn’t start the ZEN kernel again. Ultimately, the only kernel that works for me is the LTS kernel. No problems with it so far, although it wouldn’t boot on my old installation either. I don’t know if there’s been a change to the LTS kernel since then?
After updating today, everything seems to be back to normal. I also reinstalled the Zen kernel, and everything is working as it should. All three kernels (Normal, LTS, and Zen) boot normally. I’ll probably never figure out exactly what went wrong, but the problems have nothing to do with my hardware, even though it’s old. I actually like Linux for exactly that reason: it works great on older hardware. The latest kernel update solved the problem the last update caused. Interestingly, I’m running Endeavour on a really old Lenovo laptop with a 2 GHz Cuore 2 Duo from 2008 with only 4 GB of RAM, and I had no problems there, just like my 2009 MacBook Pro or my 2011 Dell Latitude. All my computers are a bit older, but they’re sufficient for what I use them for. As already mentioned, all of these machines have been running almost flawlessly for many years with the same dual-boot configuration: EndeavourOS (formerly Antergos) and MX Linux. I’d be interested to know if there have been any other posts in the forum regarding the same issue, and if anyone knows what the exact cause was. Regards!