bakelite VS erpc

If you want to explore the custom protocol approach, you might be interested in this library I wrote. It helps automate the tedious bits of building your own protocol: https://github.com/brendan0powers/bakelite

Here's a link: https://github.com/brendan0powers/bakelite An Arduino/Python example: https://github.com/brendan0powers/bakelite/tree/master/examples/arduino

I would love to see the kind of HID solution that you describe paired with a proper strongly typed message format like eRPC. In previous searches for similar messaging, I have also found things like EmbeddedProto and nanopd. I never got to dive into the details on any of these, though.

I've seen people mention Messagepack, Asn.1 and Nanopb, which are data serialization libraries. Since in your example you mention that your protocol is like {START_COMMAND} {MODE_BYTE} {MODE_LENGTH} {MODE_DATA} I suppose that what you want is not only to transmit data, but also commands. I believe that RPC (Remote Procedure Call) is what you're looking for. Unfortunately I haven't found any really mature and well-known RPC frameworks for embedded. There is eRPC, but it is not so actively maintained. The RPC module from Pigweed, a embedded toolkit from Google, looks promising but IMHO it's a work in progress (especially the documentation is quite lacking).