Which GP should I play in?

Guys, I need help if I have 6700 points and I'm in series 11, really in which category should I compete in the GPs? One day I play in champion and reach the finals, the next I can barely qualify, if I play in rival all the players make the total points in the races, I still haven't tried contender.

I don't understand how GP matchmaking works. But at this rate it is impossible for me to enter the top 10 and I do not lack coins and in the category that I play I try to improve my pieces to the maximum and use the best power-ups but my matchmaking groups in the GPS are really complicated they practically have a very high level, outside of champion and contender (which I have not yet tried) from top 1 to 20 they make the total points crazy

In champions I don't have the level to compete because I still don't have the parts of the 12 series