Here is the issue with modularity like you are planning. If a cable is loose because something bumped into it you now have a problem to chase. Even worse you now have the potential for a power cable to become completely unplugged while the host is still up and running. Having two power supplies also creates a potential for a ground loop through the data cables if things are not made correctly.

Some people have been able to use USB drives connected to a computer to create a NAS but they are also the ones hitting forums because they have issues. What you are trying to do isn’t much different. If you want to keep your hardware you would be much better off getting a case that can hold all your drives. A used server chassis would be ideal if you use a SAS card. But if you are going to upgrade anyway maybe it’s a time to make plans to make the switch sooner than later.

Do some commercial solutions for this exist that are safe, yes they also live in racks and are effectively ground connected with the cabling protected/much less likely to be bumped into.