Greetings! I beat vanilla game and both expansions a long time ago. Since then I got the separate GOTY version of the game in my library. Can I carry my save over to the GOTY edition and begin NG+? Thank you!

Yes

Yes you can.

But remember, there is no NG ++, or higher.
You can only replay once, and after that you need to start your campaign from scratch.

Before starting a new game plus,
a) Create every potion, blade oils, decoctions, and bombs.
b) Gather every schematic you can, and build the most powerful set for you playstyle.
c) Put every item you don't need from the vanilla play through, into the storage chest. You may need them in the future.
d) Complete every side quest to earn the maximum exp

Hope this helps
gogwitcher300