ARWing Pro VTOL tailsitter, how to debug FW->Q mode transitions?

I am an experienced racing/freestyle quadcopter pilot trying to have some fun with fixed wings. Last summer an AR Wing Pro with a setup issue in manual mode flying LOS turned out to be a bit too much to handle, and I turned the model into a thousand pieces of foam.

For regular fixed-wing flying I bought another model, but since the AR Wing Pro was already broken I thought ‘good for some experiments. If it breaks, oh well, it was already broken anyway’.
I made a small video about this, not mandatory to watch: ARWing Pro, the road to VTOL - YouTube

So, I added two tractor motor mounts to the front of the plane equipped with Racerstar 2508 motors and 7x4.5" props (drone parts junkbin), mounted an H743 Wing and tried to turn it into a tailsitter VTOL with some succes.

The first maiden flight with most settings at their default went fairly well except for a pilot error at the end. That was the flight displayed in the Youtube video above.
Then, I set up FFT-based harmonic notch filtering and started with the tailsitter VTOL tuning guide and set up the FF/I.

Inititally I was unable to get the plane in a stable vertical hover again; it had the tendency to flip over backwards and fly horizontally upside down. I increased elevon surface area, without any change in behaviour.
Short video of this behaviour: Nextcloud
(I disarmed as soon as it went away horizontally upside down, but when not disarming it can fly 100+ meters inverted before crashing)

Halving Q_A_RAT_PIT_P/D did not improve anything, doubling made it stable enough to be able to run the VTOL quicktune script. Control authority is still not strong enough to fight a bit of wind though, I guess that is the weak point of a nonvectored setup. But I am able to take off vertically in a qhover and transition into FW mode without issues.

The issue is transitioning back from FW->VTOL. I am getting ‘VTOL overshoot’ messages that I do not understand, and I could use a little help to figure out where to look in the logfiles.
Here is a screen capture of the logfile plotter (offtopic: what an awesome tool!) with a transition to QRTL around the 50 second mark: Nextcloud
It seems to match what I saw in the goggles.
I also tried transitioning to QLAND, QHOVER and QSTABILISE also (basicallly just playing around), behaviour is generally the same although I did have a one-time success with a transition to QLAND.

Param file: Nextcloud
Dataflash log: Nextcloud

The entire state of setup and tune is probably in an less than ideal shape (RTL is going full throttle, for example), but I am still learning. Therefore, I am less interested in ‘please fix this for me’, more in understanding what is going on and how to interpret key parameters in the logfiles.