This is a bit similar to roll your own crypto conversation. It is fun, and a great way to learn. And once done, you should do well to never deploy your own crypto in any real life application.
Friends don't let (or shouldn't at least) friends roll their own protocol over UDP either (or IP; I mean why not?). There are very few apps that have such an explicit requirements that they can't do away with TLS over any number (==2) of existing, and well known transport protocols (e.g. TCP, maybe QUIC). For presentation layer, instead of implementing their own spec, most should probably stick to Protobuf[0] (or some of it's derivatives[1]), and encode the application data into JSON.
Check. Protocol design done.