There have been quite some request for modularity like the one your asking, and it is in constructs 2 future plans for development.
what people do atm:
-Simply copy pasting of sheets / variables / objects between projects.
-Make their own templates with various game elements / objects already set up.
For event sheets I personally make sure that when I create something that is going to be reusable, that I include everything needed for that event sheet is actually on that sheet. (like variables)
This way, I only have to copy/paste the top set of variables from a sheet, and after that copy paste the rest of the entire event sheet.
For objects I use dedicated layouts, from which I can easily copy/paste the required assets.
Might not be the most gracefull, but it surely makes managing assets between projects "managable".