npubs won't work with any existing DHT because they are secp, ed25519 works great though.

Check: pkarr.nuh.dev

and github.com/nuhvi/pkarr

as for private groups, if we just use something like Pkarr, you can open a Matrix account and clients can use both your nostr key for timeline as well as Matrix for encrypted groups

for example I already publish both my npub and Matrix ID (you can find that on my Nostr timeline and Twitter bio arnazeh)