Nice shots. Looks like you've got it dialed in pretty close. The new v0.92 update is supposed to fix the accumulation problem. I've seen the several times, even on the ground, as I was adjusting the gains. After making big platform movements, you could see that the "ball" was off by 4-5 degrees, usually in the roll axis for me. Rebooting cleared up the error each time. I haven't tried v0.92 yet, but I'm glad to see they found this error.
Not sure what you can do about the yaw-roll coupling, unless you simply move the board as far away from the camera as you can. Mine is now at the back of our gimbal, above the roll servo.
Below are my last two videos. There's a bit of wind-related bounces that we are working on tuning out, but I had no problem with the HF Gimbal board holding the horizon.
Today I will try and get v0.92 loaded up.