While you could take the time to wire PKDNS to any other protocol, like Nostr, you would effectively just end up building the Pubky system itself, which we have already put a lot of work into!
It is our candid opinion that nostr's relay model is naive, and obsoleted by PKDNS + homeservers.
There is no enforced method required for encryption or private data because PKDNS is routing to data anywhere, which could mean public data or permissioned data, however your homeserver desires.
That said, what we have built is a homeserver, indexer and app interface that "speak" PKDNS, and our "semantic social graph" concepts.
We are currently in R&D for our first private data primitives that we intend to support intentionally, and deciding how forward-looking (complex) we want to be.
Regarding SimpleX specifically, it isn't something I have a lot of experience with, but one of the core architects behind Pubky's design is also a big a fan.
Aside from that, we do already have a vibe-coded e2ee chat demo here, but all of the activity and encrypted files are public in this example.