is the right answer (given the constraints) to simply set up a desktop PC in another room with DB Poweramp and a RAID 5 NAS hooked up to it to store the music, and then find some other app later that will read FLAC / WAV files off the NAS when i'm ready to stream to a DAC?
Yes, precisely. There are several programs available (Sonos is one) that will play from your NAS drive. Get a redundant set up with a couple of decent sized (4 TB?) drives. Raid 5 requires at least 3 discs; Raid one is simple mirroring that needs only two discs.