I do think that now is the time to focus on improving this version while it's experiencing a boost in popularity (and funding). A new map, etc. are nice. However, if the underlying code is indeed old, inefficient, and/or hard to scale then I think you should at some point shift focus to AU2. Better code, and all the improvements you've learned from AU1. If you want to extend the shelf life of AU2 to basically forever, letting people make/play custom maps would be the best way I think (heavily curated if you add them to the game).
If you do make AU2 or whatever, just don't be tempted to listen to the really bad players who want the game to be easier, etc. Some people just need to git gud. Thanks for the great game.
Oh, also, while this will probably affect cross-play, Steam has their own stable multiplayer API. Maybe use that as a backup?