Some of the popular middle school competitions are:
- AMC 8
- MathCounts
- MathLeague.org
- Math Kangaroo and many more…
There is a comprehensive list of math competitions here:
https://www.omegalearn.org/middle-math-contests
Lot of resources (many of them free) listed here:
https://www.omegalearn.org/middle-competition-math