You have got lots of questions in the post.
Cardas89 What information will Google have with regards to my smartphone and app use, when using the Sandboxed Play Store in a separate user profile?
This is what they will see in terms of devices: https://myaccount.google.com/device-activity and, obviously, they will know what apps are installed on your devices.
Cardas89 My impression is that the Play Store with personal Google Account is recommended primarily because of security concerns with the alternatives (F-Droid Store, Aurora Store, APK sites etc). Is that correct? If yes, how do the privacy concerns of a personal Google account stack against this?
If you are not targeted specifically, it is more secure to have Google Account with Play Store.
Cardas89 Is this correct, or is there too much doom and gloom here?
If you are targeted specifically they will get to you by sending an custom update of an app to you.
Cardas89 does that actually make a significant difference in reducing big tech influence? Or am I primarily creating a false sense of privacy for myself?
That is a very complicated question and the answer is different in each case. The general rule of thumb is that your on-line activity is traceable. They key is to make it difficult to link this activity to you.
With GrapheneOS on Pixel 7 Pro I have also found that body and physical activity sensors can actually be restricted so that Google does not know if you are walking, running, traveling in a car or public transport.
Apps access to microphone and camera is really powerful on GrapheneOS. Again, if you are targeted specifically, then I assume it is possible to connect to Pixel's mobile modem backdoor to listen in, at the end of the day this part of software is closed source so anything is possible. This is issue affect any mobile phone manufacturer who, I assume, are forced to plant this backdoor to be given certification required for mass production.
I wish you well and good luck in your journey to privacy.