After having trimmed the quad, all seems to be quite a lot more controllable.
Apart from some glitches in the pilot (who now and then panics and just closes the throttle, turning the quad into an expensive brick), and a possible glitch in stable-mode (after a while it seems to lose calibration and just drift away, fast) the quad is very, very stable in the air.
The whole device still feels somewhat twitchy, so maybe some experimentation with curves for the throttle (making it less sensitive to my inputs in the middle of its range) and making it less aggressive when it tries to get to the attitude the software tells it to.
I’ll also rebuild the quad to have the battery on top, so I can mount the small HD-camera I bought (only 15 grams!) below the frame, and to connect some more of the channels of the receiver to enable heading-hold (via the magnetometer) and possibly altitude hold (via the barometer), but the last one still has a bandwidth of 2 to 3 meters, which is about as high as I dare to fly, so that one will have to wait until I have more confidence in my capabilities to bring the quad down in a controlled matter whatever it’s position in the air.