I have been playing mania for roughly the same time, and for 7k I use my left thumb (I am left-handed). However, this is a bit inconveniant since when playing 7+1k (8k) I press the special key with my left hand too, making me use all 5 fingers of my left hand but only 3 of the right. Therefore I think it's better to use the right thumb.
But I would say definitely use only one thumb (unless you play 10k) to make things easier, since your brain builds a connection between seeing the note and moving your thumb, which I think (not a doctor nor scientist here) you theoretically improves faster; also you don't have to think every time which thumb to use.
However, at more advanced levels you might need to use two thumbs in certain 7k maps (Watch) but I highly doubt me nor you will need that very soon.Also, the general position of your body shouldn't be like a T-rex where you raise your arm up to a table; It would preferably be the most comfortable when your elbow is at around 80-90° (this might be incorrect but I find it the best), because you can utilize the strength of your upper arm and upper body (similar to playing the piano)
Regarding the position of the thumb, I just leave it the same way as when I am typing, so are my other fingers.
