mDNS is probably the way to go, UPnP is de-facto deprecated (and should be, it's awful) while mDNS is pretty well supported and tends to "just work" most of the time IME. That being said, HTTP and Websockets are a freaking horrible choice for an embedded system. SO MUCH TEXT PROCESSING. The poor little MCUs probably struggle to do that in any reasonable amount of time. Not to mention you probably want to make use of RTP/RFC3550 to get priority for realtime-sensitive traffic (maybe this is less relevant over a LAN, but I'd still at least test it in a few scenarios), something websockets/HTTP are incapable of.
oh wow, SOAP/XML is somehow worse. Websocket is a binary protocol, but you're probably slinging HTTP reqs or JSON over it. The downside is the protocol itself is stateful so you have the overhead of doing an HTTP upgrade + managing a stateful TCP connection. Idk why you would use either of those technologies for LAN-only transports, though I could see it being the best option for talking to the mothership.
I guess that means it's entirely engineering practice or maturity. That just makes me sad :(
