How to best share code between projects in Quartus Prime Standard and Quartus Prime Pro ?

Arria 10 is actually the only device family that is supported in both Standard and Pro. I don't see why you need to move to Pro given you're already used to Standard.

For Qsys (now called Platform Designer), a .qsys file from the Standard edition can be brought into Pro, but it will be converted and cannot be used again in Standard.

As for VHDL, yes, there is better VHDL2008 compatibility in Pro. To learn all the details about the differences, see the online training (it's not called Spectra-Q anymore, but all the info in there is still valid):

https://www.intel.com/content/www/us/en/programmable/support/training/course/osynqpro.html

So based on your questions, the only reason I see where you would have to move to Pro is if you need the VHDL 2008 compatibility. Otherwise, you can continue using Standard.