Upgrade platter/sub platter or cartridge on Rega p2?

You would be better off getting a better turntable - go up the Rega chain.

So in the meantime buy an Exact and keep the Elys 2 for when you sell the P2 down the road. With the Exact you don't have to worry about arm spacers for VTA.

If you had a P5/Rega Power Supply then it would be worth upgrading the subplatter etc. The Groovetracer mods are very good.

Ideally you would jump to the new generation P6/8.

I don’t have the funds to upgrade to a better table. Your idea to get the exact 2 and save the Elys 2 should I ever able to move up the rega line is a good one. 

With P2 keep in mind the cartridge height. There’s no VTA adjustment on that table.

I had good luck with Hana EL. It seems to fit perfectly.