I did try PID control (software) in fairly simple way, and it was a matter of tuning the gains as I didn't have to worry about the output which was fed to the "plant", but this one is more complex.
I'm aware of existing FOSS FCs and checking the source code is not something I can make sense out of it.
My goal when I started this project is to cut with simple projects and step up to learn concepts and gain skills, and I'm sure it'll take me long time to get a decent outcome, in terms of knowledge and final working project.
I read few of your posts and I know you're very skilled and probably this post is senseless and you're right, I'm lost in it 