- Edited
A bunch of really good questions! Let's see how good I can address them:
BlueSky I installed the PS in a separate Profile. But still is this now undermining the whole idea of GOS and ruins the privacy since Play service and framework are sniffing in the background the whole time?
It doesn't undermine the idea of GOS (otherwise it wouldn't be so deeply implemented). Google doesn't have privileged access (except optional eSIM activation in owner profile). In your example you're doing a good job at compartmentalization, so Google should only get the data other apps share over PS and GSF. If you don't depend on notification forwarding, you can also disable the profile running in the brackground.
BlueSky Is it more dangerous to install it into the profile that i actually use?
I don't think it's more "dangerous" regarding security, all apps stay sandboxed no matter the profile. You need to figure out in your threat model how important privacy towards Google is for you, since that is what you control with your decisions when and where to use Sandboxed PS. You probably need some kind of appstore in your profiles. Maybe you would entertain the idea of using Aurora Store in your owner profile to manage all apps, but push them to different user profiles where you actually use them (discussed in this thread)
BlueSky What are the pros and cons of using the sandboxed version vs Aurora?
There are many discussions on this forum about this (search "Aurora" and sort by latest). The short answer is that Play Store is more convenient (especially lately because Google is rate limiting Aurora to hell) but Aurora doesn't need PS or a Google Account from you. You should also check the other alternatives and supplements such as F-Droid (Droid-ify, Neo Store) and Obtainium. Pick what fits your use case the best, none of them are generally bad (as long as you don't use outdated apps such as Aurora Droid).
BlueSky Is aurora less secure due to Account sharing, and in case of update i read that signature spoofing could be more of a risk?
The account sharing is not a security risk to my knowledge, rather a privacy benefit. I'm not sure about signature spoofing, but I think the apks are signed by the Play Store and not Aurora (unlike apps from the F-Droid repository which are signed by F-Droid).
BlueSky It rarely could happen that i would enter credit card data into an App but it could happen if that makes it a difference.
Any app you give your credit card to is a potential risk (leaks, breaches etc), but I don't think it makes a difference where you install the app from as long as it's the most up to date version and you made sure it's the correct one (e.g. via pgp-signature when downloading via F-Droid or Obtainium). A general advice is to use virtual credit cards or even auto-disposable credit cards that are easier to destroy and recreate, and set a charge limit for them. You can generally use pseudonyms since names on credit cards rarely get verified.
Hope this helps you. Please be aware that the article you linked is quite old by now and many things have changed or been added ever since. Check out the Features, Usage, FAQ and Releases sections on the official homepage to get up to date information.
Edit: Typos