Thank you for being open to the idea. Having no personal experience distributing applications against payment, all I can draw on is my experience as a customer.

Threema recently introduced their app, which requires a paid-for license, through F-Droid using their own repository and license shop parallel to their established offer on GooglePlay.

I can only echo rchaud in assuming in theory it should be possible to sell the license and do the check independent of Google like desktop software has been doing for decades.

Whether this is something worth your time is, of course, a totally different question.

Just wanted to make you aware of this in case you weren't.