mbin is probably the closest to what you want. Its supports both threaded content (lemmy) as well as microblogging (mastodon) Here is a list of servers https://joinmbin.org/servers and interstellar is the mbin app.
If piefed decides to create an api later, I would prefer they did their own thing and try to be lemmy compatible. Looking at misskey, I think not forcing api to be compatible with the more popular service has many upsides.
Thanks. Had to make a mbin account as interstellar with lemmy account did not display any comment. Still need to find out how to use mastodon inside of it xD