Monster Energy Supercross - The Official Videogame

This can be the same custom track for all 10 or 30, it doesn't need to be 10 or 30 unique tracks.

This means, if you wish, you can find a short/easy/simple track that works well for you and complete it 30 times. Likely, you can win 30 times as well which will gain you progress towards the 50 Main event wins as well as quite a few credits, prestige, and miles on your vehicle for those achievements. Could also be used to prepare yourself for upgrades in career.

Personally, at first I mostly used the track "4" created by a gamer "Big Neily91".
I've also uploaded a few very short or simple tracks myself. I imagine they would work nicely.
One track is aptly titled "short n simple".

Unfortunately, there does not seem to be a search by text yet to find tracks, so you may just have to look through tracks to find one that seems short/easy. You can see the track itself in the background when flipping through online tracks. Once you find one, you can press right on the d pad to favorite it, making it far easier to find later by tabbing with to "My favorites".

For clarity, you do this from the main menu by:
Select: Single Player -> Single Event -> (any but I used 250 east mostly, 450 may give more $) -> Track Selection -> Online Tracks -> Find a track, or tab to My favorites.

Probably easiest Race Options:
Indoor, One Shot, Short, Very Easy, Assisted, On, Semi-Auto, Semi-Auto, On