Honestly, I think Agenda [1] had a brilliant take on this - a free version with baseline features, and a continual improvement of "Pro" features. Pay for it once, get all the current pro features plus whatever they release as pro in the next year. Then your feature set freezes at that level, and whenever you feel like they've added something worth the pro price point, you pay for the catch-up and another year.

They get to maintain one version of the app, with a reasonable number of feature flags, and users get to pay when something adds value.

[1] https://agenda.com/