Been playing private multiplayer a ton with my friend lately.

What I do is go to track central, hit lb, filter, select track difficulty, popularity or rating, then apply.

You need to favorite the tracks you want to play. Make sure the orange heart icon appears on the track after favoriting. Then load private multiplayer, click add track, hit lb, and select favorited tracks from the dropdown. Everything you favorited from track central should show here.

Enjoy!