a critical software update is required for your mac, but

#A critical software update is required for your mac, but install

But at least I could verify that my touchbar was working fine, since I was able to test it. But then I tried a simple reboot, and the issue came back again. I've tried this many times, but one time it miraculously worked, I could start up my MPB in normal mode without any issues! I set my appearance preferences and was all set to continue working.

#A critical software update is required for your mac, but install

Then I erased/formatted/first aided my entire disk and tried to install Big Sur, but still the same issue.

I then reinstalled my Big Sur installation from recovery mode (without erasing my disk), but still, the error message came up when booting. Using Apple Diagnostics to check for hardware issues also didn't help me, since there were not issues found.

This still led to the same error message. Through safe mode I also installed the updates manually. I was able to boot in safe mode, so I could at least back up my important files. Unfortunately, that didn't solve it this time. When I has this issue before, last year, I managed to solve it by attaching an ethernet cable. Retrying or shutting down and starting up again didn't help. During the reboot I got the same error message. Apparently, my MPB was updating in the background (at least, that's what I suspect, even though I always have auto installing off). As I was working, my Macbook Pro (2017-mid with touchbar) heated up, about two weeks ago.