How about 1 edit within 90sec of posting? You usually notice all your mistakes as soon as you hit post, so a short, one-time opportunity to make corrections would be ideal. And implementing it could be entirely at the client level.

When you post, the client doesn't actually send it to the relays, but keeps it in a temporary, 90sec delay queue. That window gives you time to make an edit, and after that time is up, the delayed note is automatically sent to the relays. This way, relay devs don't need to do anything to make it happen.