I was picky about the ones I feed to… community ones I feed to are adsb.fi, adsb.lol, airplanes.live, plane.watch, opensky-network, and on the commercial side, flight radar 24 and fightaware, both give premium access for feeding, which is nice.

That is neat.

I know you can set up a self-hosted Signal server, but I’m not aware of any relay/proxy type apps

yup :) Already doing that.