Since the Gog preservation project made itself that it is stable and can run a modern systems, Do I no longer need to install the ZRP mod to ensure the best experience possible with this game?

ZRP fixes a lot more issues (bugs with missions, etc...) than the preservation project did, and so it is still recommended for a vanilla (as ZRP does not modify the balance or anything like that, unless you modify it yourself with the zrp configurator) run in Shadow of Chernobyl without trouble.

Here's a (big) list of the bugs the ZRP is fixing if you're curious :
https://www.metacognix.com/stlkrsoc/BugsInSTALKER.html