Lens changing technique is a very important factor indeed. Holding down the camera so that nothing can fall in during the procedure is very helpful. So is removing dust from the lens' rear element before attaching (I use a Lenspen for that).
"Hard dust", as you call it, comes from heating up the sensor. It can be reduced significantly by turning off the camera whenever you don't use it, including between shots.
Developing dust preventing habits can be very effective in reducing the need for cleaning.
Yes, for sure ... the best solution is not to get dust in in the first place. I didn't know about the dust getting 'burnt in' by the sensor getting hot.
Regarding the gel stick, this (from PhotographyLife) is pretty scary:
"Sony A7 II, A7R II and A7S II advisory: Please be extremely careful when using the product with the A7 II, A7R II and A7S II cameras. Since the cameras have in-body stabilization, the sensor will move during cleaning. Instead of picking the Sensor Gel Stick straight up, we recommend to detach it gently from the side and be attentive during the process of cleaning, so that the IBIS system does not get moved too much. We have tested the product on the A7 II and A7R II successfully, but one must perform the procedure with extreme care on such moving sensors. If the gel sticks a bit too much, please detach it slowly from the side instead of moving it straight up, as shown in the videos."
Not at all sure I want to go there (I have an A7II and A7RII). Have you used the gel stick on any of these cameras with the IBIS? I suppose one option would be to do the gel-stick cleaning with the camera powered on.
Beginning to sound like camera sensor clean, air ... and occasional wet clean.