QUICKSILVER VS inav

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

https://github.com/iNavFlight/inav/blob/master/docs/Failsafe.md Looks like something wrong with your failsafes

iNav is not hours of setup. Heck, if you are experienced with BF then it's barely an hour. Look at this guide. It is not 300 pages long. Basic setup you only need to do once (always save your CLI DIFF after setup in case you need to go back). One flight to run autotune. Then you are done.

https://github.com/iNavFlight/inav/wiki/Supported-boards (I feel like iNav can be installed on a lot more than those 5 mentioned on this page)

Take a look at the stick commands chart half way down this page.