I have the Silverstone GD-08 case that can fit 12 hard drives. It’s an inexpensive solution for a dedicated htpc/media server. Even managed to fit in an RX 6600 so my wife can play low spec games on the tv.

Then all you need is a pcie expansion card as others suggested.

Most of the original d&d games, baldur’s gate 1 &2 and neverwinter nights etc, have Linux native versions and go on sale for a couple of dollars. You can easily sink 100s of hours into them, especially neverwinter which has amazing community made campaigns.