For GUI, I've been very happy with grpcui-web[0]. It really highlights the strengths of GRPC: you get a full list of available operations (either from the server directly if it exposes metadata, or by pointing to the .proto file if not), since everything is strongly typed you get client-side field validation and custom controls e.g. a date picker for timestamp types or drop-down for enums. The experience is a lot better than copy & pasting from docs for trying out JSON-HTTP APIs.
In general though I agree devex for gRPC is poor. I primarily work with the Python and Go APIs and they can be very frustrating. Basic operations like "turn pbtypes.Timestamp into a Python datetime or Go time.Time" are poorly documented and not obvious. proto3 removing `optional` was a flub and then adding it back was an even bigger flub; I have a bunch of protos which rely on the `google.protobuf.Int64Value` wrapper types which can never be changed (without a massive migration which I'm not doing). And even figuring out how to build the stuff consistently is a challenge! I had to build out a centralized protobuf build server that could use consistent versions of protoc plus the appropriate proto-gen plugins. I think buf.build basically does this now but they didn't exist then.
In general though I agree devex for gRPC is poor. I primarily work with the Python and Go APIs and they can be very frustrating. Basic operations like "turn pbtypes.Timestamp into a Python datetime or Go time.Time" are poorly documented and not obvious. proto3 removing `optional` was a flub and then adding it back was an even bigger flub; I have a bunch of protos which rely on the `google.protobuf.Int64Value` wrapper types which can never be changed (without a massive migration which I'm not doing). And even figuring out how to build the stuff consistently is a challenge! I had to build out a centralized protobuf build server that could use consistent versions of protoc plus the appropriate proto-gen plugins. I think buf.build basically does this now but they didn't exist then.
[0] https://github.com/fullstorydev/grpcui