I might be biased since I've only ever really done Kumon and Mathnasium as a child and those aren't the best options if you want in depth and higher level difficulty, but I've been doing spirit of math for a while now and I think it's a really good program and that you should consider taking it? They have courses from kindergarten to 11, 11 being equivalent to about a first year or second year university math course (according to their website) and I think they have really good problems (lots of experience just from doing these problems) and teach you really useful skills.
I wish this was sponsered but it isn't lol
tl;dr Spirit of math offers good courses (right know it's online but if there wasn't a pandemic it's in person) and all you need is like a B in all subjects or something like that to get in. Good topics + experience. There are grades but they don't matter :)