Most users will expect to use the @ format especially when tagging users, simply because that’s what 10+ years of social media have taught us.

I think Damus must do the first two things you suggested, but with a bare identifier it adds nostr: automatically. In these screenshots I used the naddr provided by Habla and the noteID provided by Damus for the same article (Will’s nostrdb article). In the first test Damus added nostr: automatically to the naddr, but didn’t render a preview or link. In the other 2 tests I used the note ID format with and without the @ and in both cases it rendered a preview. The Nostur client doesn’t render a preview for naddr, but it does make it clickable and opens the corresponding article in the client, which I think is at least an acceptable fallback.

Just makes sense from a user side that if you tag someone or a note in your client, people in other clients will be able to see who/what you tagged.