Bathtub faucet trickles out when single handle turned to hot side. Sink hot water is OK. No access panel either.

Most likely debris in the hot side port. Turn water off at the house, remove cartridge, flush port by putting a rag over the opening and have someone turn on the water momentarily to flush the base port. Then make sure the cartridge port is clear. If you find a lot of small white pieces, you may have a deteriorating dip tube.

Your temperature balancing device has most-likely gone bad. Replace the cartridge.

Milo