On the Mac App Store, there isn't a practical way to charge for updates. You could release entirely new apps, but then upgrading is a pain for users, and you lose your ranking and reviews -- and it's difficult to charge an upgrade fee. You can gate-keep features with In-App-Purchases the way Agenda does, but then you're giving away bug fixes and polish on the app for free forever, and that's probably 80% of your development time. Plus, I would imagine the upgrade rate on that model is pretty low, since most users won't care about fringe features being added.

If you really want to offer perpetual licenses with paid updates, since you're a desktop app, you can roll your own licensing system and use FastSpring/Paddle/etc. It's a fair model, but it's a lot of work. It may be worth it depending on your audience - e.g. developers tend to care a lot about this stuff.

Selling this as a subscription is probably the best path if you can stomach the initial ire of users that don't like that model. Depending on your price point, you could consider a 4x-5x multiplier for a lifetime option if you want to try and keep some of them. Yes, you will lose some users that might have paid for a major version, but you'll probably make that up with the recurring revenue from less price-sensitive users.

Best of luck. I know this can be agonizing and there's no easy answer here.