Simply install debian 12.5 again, the easiest choice.
Good choice.
Install linux mint, so I get ubuntu but without them throwing their subscription services down my throat. I’m unsure about other advantages, as ubuntu is debian based, maybe the more frequent program updates? Kernels are also updated more often than with debian as far as I know. Do you know of other advantages?
There’s LMDE, Linux Mint Debian version.
Go for FreeBSD: this might require a learning curve, because this is an OS I’ve never used. Are commands that different from debian?
Yes, commands are different (For example ifconfig and not ip. And watch on Linux is something different on FreeBSD) and you can expect several things to not work out of the box. Also, mounting removable devices is different. Documentation is very good though unless your reached a niche problem. I’d suggest to first toy around with FreeBSD in a VM (Qemu or VirtualBox) if you want to sneak preview it and learn more.