> So when playing against really good players, way above my level, I could win sometimes (or allmost) simply by making unconventional (dumb by normal strategy) moves in the beginning - resulting in a non standard game where I could apply pressure in a way the opponent was not prepared for (also they underestimated me after the initial dumb moves).

IIRC Magnus Carlsen is said to do something like this as well - he'll play opening lines that are known to be theoretically suboptimal to take his opponent out of prep, after which he can rely on his own prep/skills to give him better winning chances.