If you're already on nostr then it's unclear how exactly the newsletter should work. You can already follow npubs and hashtags and post are delivered in real-time. Paid subscriptions is a different matter though.

We had email in mind for people outside of Nostr - you share your posts with friends and they can subscribe by email first, until your purple-pill them. This could also work for people wishing to migrate from substack/medium/etc - import subscribers from there and start publishing on nostr - existing subs get updates by email.