I am a Computer Hardware professional. I started working with computer technology in the early eighties. I have seen the evolution of technology starting with closed platforms like the game console era and then the move toward open platforms like the Home Computer Golden Age. In the last 5 or 10 years, I have witnessed technology changes that are slowly moving away from open hardware designs towards hardware that is locked down and can’t be modified by the user.
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.