Late 2015 27" iMac crashes on login after 12.6.4 upgrade

[Soylent Yellow]

My Late 2015 27" iMac with an external T5 SSD crashes on boot after the 12.6.4 upgrade.

I have had boot problems before, a 'normal' boot takes ages, but finishes fine. As long as I let the system sleep instead of switch off, it stays running just fine.
Occasionally, after a power cycle or a system update, I had the same boot problems:
1) Takes a long time to login after boot (hangs halfway for ages)
2) When boot progress bar starts moving again, I get the 'Your Mac will be restarted because of a problem. Press Any key" white text on black screen.

I managed to solve that every time by:
3) Boot from recovery
4) Reinstall MacOS (which takes hours)
5) Login again

But now I get the same crashes, even after MacOS reinstall.

Does anybody have solutions? I tried to reinstall 12.6.3 from the Time Machine back-up, but I am not allowed to downgrade…

 

[Honza1]

Solution? Unlikely anyone would now what exactly is wrong with specifically this system. But I had "hanging" issues previously when my SSD was dying on my MBP. Result was corruption of data which caused system to be unstable and very slow. I mean very slow... Kind of similar to what you see, but my system died totally within days.
Since you seem to be running from external SSD (if I understand correctly), did you try different boot drive? You could get different drive, install macOS on it and see if that works. If the external SSD is not boot drive, try starting without it.

 

[Soylent Yellow]

But I had "hanging" issues previously when my SSD was dying on my MBP.

I've had the 'hanging' issues since Big Sur, on two different SSDs. So the 'normal' slow boot seems to me a mismatch between MacOS and external SSDs.

That first SSD indeed died one day with mount failures, but back then the solution was simple. Buy a new SSD, format, install MacOS, and restore from Time Machine Backup.

Now however booting in Recovery mode refuses to restore from the Time Machine Backup, because the SSD has a more recent MacOS (12.6.4) than the latest Time Machine Backup (12.6.3).
I can't downgrade the the SSD to 12.6.3 (installer won't allow it) and can't wipe it and start from Sierra either because that won't install over disks that had the AFPS partioning on them (I tried that with an external HD I had lying around).

The whole situations irks me a lot because I thought I could always restore from a recent Time Machine backup, but right now the only option open seems to be to wipe the SSD, re-install 12.6.4, hope for the best, and restore the Time Machine backup through migration assistent. That sucks, and is not the first time Time Machine has let me down (previously had issues with case sensitivity). You'd think that the default back-up mechanism would be bullet-proof, but it is not.