Here’s a previous post showing the HP290 doing 21 transcodes. The G4 should have an equal or better processor. Also, these machines use very little power compared to a gaming-rig setup.
You can also move Tautulli, Sonarr, etc, everything all over to the machine. I recently migrated my server from my Windows gaming rig (very similar to yours) to the G4 with Linux Mint + docker containers for everything. I have a NAS for storage though.