Have you looked into ArduPilot?
I have a lengthy notes file with exact settings required to make ArduPilot work well with a small quad. The result is a great experience, but getting there was not!
Also of note for ease: The ArduPilot code base is a mess compared to PX4's.
There are easier ways, like using an existing config file someone posted on AP forum or rcgroups..
It’s not made specifically for small quads and it will never fly as well as BF or even iNav. I wouldn’t expect an agricultural drone to support turtle mode, so no air mode by default shouldn’t be surprising :)
Re code base: AP supports LUA so for small mods you won’t have to change the code base at all, compared to PX4..