I'm all onboard with client-side no-trusting-servers we-can-build-our-own-thank-you-very-much.

Simplex looks like it provides good privacy. I haven't reviewed it and I don't know if you need to trust the mix network or can specify your own or whatnot. But the pieces that a privacy solution would need seem to be there so it certainly could be very good.

I wonder however that if you can send URLs over simplex and if the person clicks it you have exposed them. That wouldn't be a simplex bug, but it points out that privacy and security require closing all the holes/leaks, not just the ones you can think of right now.

We did something with sligntly less private with NIP-59, NIP-17, NIP-44, but the relay sees your client drop the giftwrap on to it, so it still can't give you ideal privacy.  Perfect privacy is just very hard without VPN/Tor.