Ok. If I remember correctly the paid version is via Inapp-abo. Right? So you install the free of charge version and pay it via subscription within the free app?
In this case you are out of luck. This is due to MicroG does not support (and will not support) the needed APIs from Google Play Services since this would become a license violation against Google. I face the very same problem with my navigational app ‘Kurviger’. I can install the free of charge version but to receive the pro version I would need an Inn-app abonnement which does not work.
I actually just recently switched back from /e/ to Windows 10 Mobile.
The problem wasn’t /e/OS (though–to be brutally honest–deGoogled Android is still Android, which I find to be a far inferior experience when compared to Windows 10 Mobile), the problem was with my phone’s hardware, which was a lemon to begin with.
Even though it is no longer supported, I am so much happier on Windows 10 Mobile. The user interface is better than Android and iOS and without a huge library of apps available to me, I find that I am able to focus more on what matters. I am not a slave to my device.
If my Window 10 Mobile becomes completely unusable I may consider going back to /e/OS
Microsoft is no longer offering support for Windows 10 Mobile, which means that if something goes wrong, Microsoft will not help you resolve the problem. That said, there had been a lot posted to support forums (be they run by Microsoft or run by third parties) about Windows 10 Mobile prior to support ending and most of these forum posts have not be taken down, so if you need to find information about how to resolve an issue, you can likely find it.
None of this means that devices running Windows 10 Mobile no longer work. There is some functionality that is cloud-based that has been stripped out. Examples include Cortana’s ability to respond to most voice commands (though, amazingly Cortana still responds to telephone and SMS commands over car Bluetooth) and Groove streaming from OneDrive. Otherwise, it works briliantly.