Ade, I'm glad you enjoyed the videos. My impression of the cheap gimbal is that it is really quite decent in terms of stability, and fairly well made for the price. I think the main thing you get with higher-end gimbals, other than presumed better quality and perhaps a third axis, is a more compact design. This one does the job but it sprouts wires all over and looks aerodynamic as a rocking chair. That doesn't matter much for slow flight though.
Other than a couple of points where it looks like you tilted too far for the camera, I'd say it was amazingly stable for the price; and as you say, for a quadcopter type vehicle, high speed isn't essential... I really liked the FPV facility too; where I am, near the docks, I reckon there's some great footage to be had, flying around the bridges and whatnot. I'll maybe have to add floats to mine, in case of a water landing!
The main difference between what I want to do with mine and most drones (including yours), I want the "stable level flight" thing to be 100% automatic; I will send digital commands such as "ascend", "descend", bank or rotate left/right; and let the flight computer work out everything else. I don't know if such a flight computer exists.... so I may be trying to get my head around some maths when it comes down to it... I've already butchered an old RC set to fit an Arduino (which will read the stick inputs & translate these into the relevant commands, which will be sent over a digital stream. If I can incorporate GPS and collision avoidance as well, then it shouldn't be too much of a step to add "return home" functionality, e.g. if it loses signal from the transmitter, or if the batteries are getting a bit low. Fun stuff!