After finishing those initial items, a few specific next items I would recommend are below:
1. In one of Piapro NT's menus, there should be a manual. If you haven't read it yet, it should be helpful for understanding how to use the software, and especially for NT's effectors (like Vocal Drive, etc.), which are similar to but also different from the Vocaloid parameters used in Piapro Studio.
2.
The General Music Theory resource
Gives a list of places with more information about music theory.
3. Our list of
Tuning Resources
This provides a list of places with lots of info about making your synth pronounce words as you desire. It was written with Vocaloid/Piapro Studio in mind, but it should still be useful for the most part.
I'd definitely recommend looking through the other items in the Resources section, though. There's a lot of really useful stuff in there; definitely more than I am readily aware of offhand.
I'd also encourage opening up NT and trying to do something. Either a cover or an original song is fine. A lot of the things you end up learning are going to be when you are trying to do something and realize that you don't know how, and at that point you end up gradually expanding your knowledge as you search for/ask about how to do specific things.