I am brand new to the speedrunning community around this game but I've been playing the game for nearly 10 years now. I was wondering what categories would be best for getting started as a part of this community?

Honestly, do what you feel. I am a beginner also, and my first ever speedrun in this game was Mushroom Cup - No Skips. I suggest that to you, but that might not be your favorite. Choose one that you like, and try your best to improve.

yeah run what you like with fun and try to improve your game in a way you want, each one prefer something diferent and i dont think there is any category better than other to "start speedrunning" in general

(i feel like this is said a lot in speedrunning forums lol but true)

i mean the prefer of the user above is mushroom cup to start speedrunning, but that depends of the person you could discuss mushroom cup having probably the "easiest tracks" but you specified having a lot of experience in this game so yeah

Time Trials are the best to get started with the game, improving driving, and learning overall tech. If you're curious you can compete in TTs over at https://www.mkwpp.com