The part about the middleboxes worries me more than mitm. If you're going to have a custom protocol, just make a whole new protocol. No need to call it "tcp-esque". We've already got quic, we just need to improve it and make it a 4th layer protocol. Give it a trendy name and make up some marketing BS about how it revolutionizes the internet, and then let middlebox vendors use its industry name to sell new gear. The special interests get rich and we get to offload implementing custom protocols and crypto to the operating system, with some glue to let the application do complex auth.