I have no experience with this, but forum search says …
Installing the genuine Google services (via Open GApps) with microG being present doesn’t go well together.
As for the standalone install of the Play Store not working … It’s possible /e/ degoogled something the Play Store App still needs in Android apart from microG mimicking Google services.
If your in-app-purchases are important to you, I guess you would have more luck with LineageOS.