The problem you encounter isn't that hard to figure out.
EVERY little bit of "forward movement while IDLE" also shows the amount the vessel will lose altitude in that direction = "nose kissing the ground after a while" syndrome.
A "forward-drift while IDLE" of 0.1 is absolutely fine! At 0.2 you should think about adjusting vtols/thrusters, IF it is a HEAVY vessel. Lighter vessels are still fine. But 0.3 has to be worked on.
This means that your upward-thrusters at the BACK are too strong (too big) OR placed a bit too far at the back! Can be solved by moving them A BIT forward AND/OR resizing them by 0.1 increments (X and Y). The exact same can be done if the vessel is drifting backwards while IDLE. Just the other way around, of course.
DON'T play around with changing the ANGLES of ANY upward-thrusters/Vtols!
IF one feels the need to do so, then NO MORE than 1-3 degrees. Everything else will solve NOTHING and will lead to even harder to fix trouble. Like in your original version!
FORGET this "helicopter nose down flying" nonsense, which only works in MOVIES, but NEITHER in real life, NOR in this game! Helicopters in real life can do a so called "emergency start", which can lead to the nose pointing down in an up to 45 degrees angle. But NO helicopter in this universe can fly this way for longer than a few seconds, or it will simply crash!
Hope this sheds some light on this issue.