Owner of Redux Framework here. I'm actually jealous you've had support from Automattic at all, we've had quite the cold shoulder despite our massive success.

The donation route just doesn't work. Not going to lie, it doesn't. The developer market is poor. And it rotates so quickly it's hard to get anything out of it.

Take Redux for example; $56+ million in known revenue on Envato alone (https://redux.io/showcase/envato-market/), and less than $7,000 a year in donations and subscriptions combined. That is NOT sustainable. My suggestion, find a way to make a pro version and stop working on features for the base. Freemium is the only way to succeed in the WordPress market, especially when you have a developer-focused market.