I booted up an old MacBook Pro and downloaded a Debian installer image, put it on a stick, moved that one to the old laptop, booted from it, entered the rescue system, which allowed me to decrypt the disk and all that, and finally to move the libcrypt1 files to the right place! And then I rebooted the laptop and I was able to log in. I am amazed. Running sudo apt --fix-broken install right now. We'll see how it goes.
I think I managed to migrate from PureOS to Debian Bookworm! Yay! The system is still missing packages and all that, but I think I'm going to keep it. Phew!
https://alexschroeder.ch/view/2023-10-18-from-pureos-to-debian