There is no Google Play authentication, like you describe, for APKs that require one time fee. Sync for reddit (Dev or Pro) had no subscription model. That is precisely why online verification based licensing is different and used by very few apps, since it is harder to implement.

Ljdawson actually seems to have gone the subscription route for Sync for Lemmy, precisely because of what I have described in this discussion, and because of smaller Lemmy userbase being opposite of Reddit’s larger userbase compensating for individual lower license fees.