Hej,
it seems kinda weird that a security-related application has a dependency on Google's Play Store. I do understand that this makes the whole selling stuff easier, but than at least make it possible to run it in Pro mode without having all the Google toolchain installed on your phone.
For instance, I run Copperhead OS for a more secure phone, without all the Google/Amazon etc. stuff. However, this also means that I can't unlock my previously bought Pro version of Enpass. Other apps have a similar selling model but still are able to unlock the Pro version without requiring Google applications, e.g., AquaMail Pro (highly recommended).
Feature Request: Make Pro version available without requiring Google tools installed!
Possible idea would be to give out (on request) license files signed by a PGP key owned by that you could verify in the App.