> But it still tightly couples itself to other parts of HTTP, particularly HTTP 3

Incorrect. In fact, gRPC's primary implementation is HTTP/2 not /3. There are alternative implementation on top of QUIC, and other folks have implemented private transport implementations on top of high performance non-Ethernet networking.

The biggest I'd seen with proxies is that they didn't/don't fully implement HTTP/2 spec, in particular "trailers" which gRPC over HTTP/2 uses to transfer status code of each call.