I'm sure that would be a lot easier (unless FreeMINT has
introduced endianness issues).
It was. But of course I had to prepare another initrd because the default one didn't contain ext4.ko and jbd2.ko. :)
So, after that I was finally able to finish the installation but oh dear, was it a long one. Against Debian 3.1, Debian 12's netinst.iso is twice as big but the installation time was 30 minutes vs. ... I don't know, several hours (I let it run during the night). And that was with TT RAM enabled!
Another hiccup was booting the installed system: since the default kernel doesn't contain pata_falcon.ko, libata.ko, ext4.ko and jbd2.ko, I had to boot into the installer again, take initrd.img from my installed partition, copy to FreeMiNT's ext2 partition, then copy it to TOS GEMDOS partition, then add -r to bootargs (unlike 2.2.25) and after that I was able to see the login prompt after a long series of debug messages. Funny to see systemd on my Falcon. :)
Could I log into the system after all? Of course not. :) This took another couple of hours: by default, Debian 12 uses yescrypt for password hashes. yescrypt is so heavy that even 60s is not enough for verifying my password so after entering login+password I was welcomed with ... a restarted login prompt because the kernel killed the login process for taking 100% CPU time for one minute. :)
So a happy end but oh dear, Debian 3.1 was a breeze in comparison to this. I have one last item on my TODO and that is trying a build from Geert's branch so I can see my NetUSBee alive.
P.S. I could also observe a few minor oddities like "shutdown" actually not shutting down the CT60 or that after a while (that happened in both the installer and the installed system), IKBD packets are interpreted somewhat... weirdly. For instance UP arrow doesn't work anymore, I have to use INSERT. shell history's UP arrow also doesn't work and if I use CTRL+P, I have to go RIGHT if I want to move the cursor LEFT.