Hello and welcome aboard.
1: Using Google (overall) and google apps wouldn't compromise the GOS security but will definitely compromise your anonymity.
If you install gservices and login to play store (any other gapp) with your previous email you will be identified and tracked by google as on any other device.
2: Google apps running in owner profile aren't sandboxes as such. But if you create and use google services and google apps in a separate profile, you can achieve some degree of sandboxing functionality.
3: It depends, in my personal case, I never let my GOS device to even smell google, meta, MS etc crape. There are fantastic FOSS alternatives out there.
That being said, your choices will mainly depend on your threat perception and the purpose. On a side note, play around with your phone for a week without Google, try different apps and services. At the end when you are comfortable, do a factory reset and start clean.
Good luck.