inav VS ardupilot

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.

They are widely used in narrow circles :)

Two of (arguably the best) open source RC aircraft flight controllers (ArduPilot and PX4) are using extended Kalman filters in their state estimators (essentially sensor fusion that provides attitude/position estimate):

https://github.com/ArduPilot/ardupilot/tree/master/libraries...

https://github.com/PX4/PX4-Autopilot/blob/main/src/modules/e...

I'm not that familiar with cleanflight/betaflight/inav scene to know what the FPV racer flight controllers use.

2. https://ardupilot.org/

Trimble also has some nice stuff, I think the BD940 is interesting. I do not know if it's recommended or not, but Ardupilot has a driver for the Septentrio GPS (https://github.com/ArduPilot/ardupilot/blob/master/libraries/AP_GPS/AP_GPS_SBF.cpp), as well for Trimble GPS (https://github.com/ArduPilot/ardupilot/blob/master/libraries/AP_GPS/AP_GPS_GSOF.cpp). So people have used these brands for drones.

For programmable open source autopilot: Arduino > Ardupilot (https://ardupilot.org).

You might also want to look at something like Ardupilot

I'd like to emphasize the reference to Ardupilot bc there is a lot of material already developed there: a lot about navigation and guidance from UAVs, and the low-level stuff. Check this out