nostr:npub1vs8prwe75wsj0saedpfvzlp2x8rveuavczt77crp5455tfmpaftqva72rn nostr:npub1xzmt2d95f7watzyqktl982d7jfe5ptktusv4j20csz8nxxgmhq6q30eykm 

I think this might be a limitation of the #ActivityPub standard itself.

It uses an @ <username> @ <instance> structure, and for this approach you might need a third identifier on top of it.