I recently upgraded from a P7 to a P9a and as previously, I opted to install GOS’s sandboxed Play Services to access Google Play as opposed to using Aurora. On my P7 this worked well and I did not notice any invasive background behavior, however this is not the case on the P9a. For example:

After I signed into my Google account, Google began auto-downloading without my permission all kinds of Google apps like Wallet, Gemini, Home, System Intelligence, etc., and asking me for install confirmation.
Google was able to track me across the web in my default Vanadium browser. I was getting popups at the bottom of Vanadium saying "Sign in to cnn.com [or whatever website] with google.com" with a button to select "Continue as [my google account name]." After doing some troubleshooting, it appears Google installed cookies/trackers into Vanadium without my knowledge. When I attempted to login to my Google Play account on the 9a, Google sent a confirmation number from my old P7 to my P9a via Vanadium to confirm I was the legitimate account holder of the Google account, and this is apparently how Google was able to track me in Vanadium. I have since deleted all my cookies in Vanadium and haven't again experienced this issue.
Once, when switching into airplane mode and turning WiFi back on, I received a notification that directed me to a Google “Pixel Phone Help” page with an article titled, “Keep your Android’s wireless connections on in Airplane mode.”
One additional thing I have not personally experienced, but came across on this forum is someone claiming the Google Gemini app is able to access the Internet even though network permissions have been denied.

All this is very troubling as it seems Google was/is embedded into my phone in ways I never intended or expected and it makes me very reluctant to continue using sandboxed Play Services and just go back to using Aurora.