You probably already know, but FoundryVTT is pretty fantastic. The software/base VTT itself is not open source, but the rule systems and modules (QOL add-ons) mostly are. I self host my own server.

Have one unraid lifetime license, just paid for a second license yearly for my backup off-site machine. Been using truenas on the off-site, but its primary storage is USB which I didn’t realize truenas isn’t fond of. Ah well. It’ll be a little easier this way, anywho.