Could it be that with a certain sensitivity you have a very good aim all at once and it stays when you use it or does one get a very good aim only with practice?
With practice but if you have a good sensitivity it will get better.
3.33 dpi for your mouse should be fine.
Should I just try it out and use the sensitivity that my aim is best for?
Yes, that's how I did mine is eight, nine or seven, but I build very quickly and don't go so well with slow so building
So in the game setting