Might not be the best advice here, but I was just watching a 4k HDR10 movie with 5.1 DTS Surround sound on my Raspberry Pi running LibreELEC streaming from my Synology NAS. To be fair, the audio is set to pass through and is decoded by my AVR, but video is decoded on Pi and I did not experience a single stutter or frame drop in the entire movie. So configs like these are definitely overkill for an HTPC. Modern day processors are quite powerful and decoding 4K video is not that much of an ask.
You should also take into consideration the heat generated and power consumption if you're going to be running this setup often. AVR and projector themselves run quite hot. So overall, you'll be putting a lot of load on the AC.