Yes the PC will be unattended for many hours at a time.

Check out usbguard, whitelist your own devices, preferably no USB drives.
The ArchWiki has a good article on it. I don't use arch, btw.
Other than that, maybe check every time you get back to the machine that your keyboard still directly connects to the machine, without an intermediate device (yes, even with usbguard), but at that point maybe just always keep your keyboard with yourself because a really motivated attacker could just place some kind of keylogger inside of it.

Maybe also look into hardening your lock screen, or finding a simpler one that's perhaps more security oriented.
Maybe it's not the case anymore, but once it was possible to unlock a locked account by crashing the screen locker. Possibly that's not the case anymore, because I remember having to type loginctl something in a new virtual terminal when my lock screen crashed after an update gone bad, but i wanted to make you aware so you can check if you are affected.