QUICKSILVER VS ExpressLRS

To add to this, most of the time one doesn't need to hack on the drone firmware itself. Just configuring/hacking the mission planner alone would go a long way: https://ardupilot.org/planner/

But if you're interested in more readable codebases without too much legacy headaches:

https://github.com/BossHobby/QUICKSILVER (for indoor drones mostly)

And for even less functionality but a good learning tool kind of codebase:

https://github.com/nickrehm/dRehmFlight

Just put the firmware builds under https://github.com/ExpressLRS/ExpressLRS/releases

I didn't really find any reference material for setup, you can see it mentioned at #2060 here.

For receiver Zlyden3D said flashing 2.0 then 3 through Betaflight passthrough worked for them.

I think the controller is only going to have PPM output at best. There's a feature request for ppm input marked "not going to happen", so I think you are out of luck. https://github.com/ExpressLRS/ExpressLRS/issues/421

All you need to do is start sending Channels packets out a UART at 115200 baud and the TX will fire up and start sending them at whatever packet rate it is already configured for. You'll need to disable UART inversion on the TX though because external modules are half-duplex inverted (stupid legacy stuff!) The protocol I've documented here: https://github.com/ExpressLRS/ExpressLRS/wiki/CRSF-Protocol