Gunfire Reborn. It's a roguelite FPS that is fun to play with friends. It's very jumpy shooty. I really dig the handling in it, and there are lots of weapons and abilities and play styles and due to the rogue like nature of it, you get to experiment with lots of stuff. It's lots of fun, but not super deep.
Phasmaphobia. Been playing this one for years somehow. It's a ghost hunting game but you don't actually hunt ghosts, you collect evidence to identify the type of ghost that is haunting a place. The ghosts will kill you though. It no longer makes me scream like it used to, but I still get chills from it. If you're looking to soil your underwear, it's a good time.
It look like Nvme riser cables exist, so your mad scientist approach should still be doable, but you would have to continue doing mad scientist shit.
Nvme riser going to the front of the case, maybe even the top, and then get one of those rubber nipple nubs that exist for holding nvme drives in place, and bam, you can swap the drives pretty easily.
Very niche requirement, but to each their own.
You could just get one big drive and partition it to have multiple OS or whatever it is you need. Then pick which one you want to boot from when you start up. Did they get rid of that ability? I haven't messed with anything like that in years.