You will be able to use up to 16 audio outputs with version 3.8. It will be 8 stereo tracks where each track can be routed to a mono or stereo output. An output can also be shared by multiple tracks. So it will work for any setup between 2 and 16 outputs. If you pan your stereo tracks accordingly you could have up to 16 independent tracks feeding your mixing desk.
Regarding the tone generator: Interesting idea. I might consider that for version 4. But to be honest, according to the metrics I get from Apple, last week the app has been started 21000 times and crashed only 29 times. That means when you use it daily it will crash about once every 4 years for you. Not sure if it is worth putting effort into this automatic failover. I'd rather keep my focus on stability.
Oh, and as an added bonus, you will be able to change speed and pitch of multitrack songs (when using 8 tracks and crossfades you might need an M1 iPad though to stay free of audible glitches).