drinkablederanged I can't really answer this. You need to define security and privacy in your threat model, and on GOS everything is more secure and private than on AOSP or Pixel OS by default.
There are things that Sandboxed Google Play Services can do to enable apps to see more stuff about you and also Google will see some stuff, but I'm no expert on the details. My general understanding is that you limit the privacy invasion of Google and other apps substantially and most people wouldn't worry about privacy with Sandboxed Google Play.
If you can find a way to live without the notifications find alternatives, this is of course the privacy cherry on top. For example: I use Tutanota instead of Protonmail exactly because I want notifications on my private GOS device but no Google Play Services, and Protonmail depends on GSF for notifications unfortunately.
So in short: With your idea of segregating apps over different user profiles and only installing Sandboxed Play in some of them, you should be within most thread models on this forum (maybe you're even overdoing it for your own needs). In the end only you will know, you can check out privacyguides for basic threat modeling if you're not sure where to start.