dryfter I can recommend this experiment: Order a 'Google Takeout' of your data from your logged in Sandboxed Google Play.
In that dialog, you'll get to choose how large the delivered chunks will be packaged as prior to you receiving them as downloads. The smallest size you can select is one gigabyte.
Every time I've tested this the delivery arrives as a zip, which after decompressing is under 500 kilobytes, and contains reports of the entries containing data from my usage, in my case almost only empty results. Hundreds upon hundreds of entries about various products and services reporting 'no data found'.
Of course this still isn't proof of abscence of collected data, of course there's still a possibility that 'they' could be disclosing only the null results to obscure their capabilities, but the scale I believe is telling me something about how scarce the collection have been when using Sandboxed Google Play.
I think the gap between the design environment for apps and the Gos strictness is much much larger than how it looks, since the apps permissions listings are the same regardless of OS, and Sandboxed Google Play is the same Google Play that's so utterly invasive in other OS:es.
Also, I've increasingly been opening up the permissions for both my phone and Sandboxed Google Play, just to get as much compatibility as possible when testing banking apps. And a very conservative guess is that I've well passed a thousand test-installs of invasive apps using Sandboxed Google Play, with an exception in RethinkDNS for each of them, and all their permissions allowed, and with the exploit protections turned off, then running them in Owner.
Still, a takeout arrives as less than 500kb decompressed. And Play still don't show me localised ads, nor personalised ads even when I've toggled these ON for testing.
Again, I cannot prove a negative, but I find it a bit remarkable how little Google appears to be collecting from Sandboxed Google Play.
Please note I'm not affiliated with the GrapheneOS project.