>>363271923
I was in the same position and I just looked up a bunch of the smaller new tutorials for avatars. The work you did/do for porting over an MMD still remains mostly the same, if you're still rocking MMD. The Unity side is a bit different with avatar 3.0 so that's where the new tutorials come in. I followed stuff like on this guy's channel:
>https://www.youtube.com/c/Sippbox/videos
Also I read VRChat's documentation on avatar 3.0. It might be confusing at first but you get used to it and understand it the more you play with it yourself. 

This is also useful if you have FBT or run into any IK problems with your body in-game. 
>https://www.youtube.com/watch?v=2sfTEBAl8sA