Consider: (A) sell it once and customers own it forever, included in the price is (B) all updates for one year since date of purchase. This means customers don't have to wait for a new release, they can just buy whenever and receive a year of updates. If after the year of updates the new features are worthy of another purchase, they can buy again. Feel free to give discounts to customers from the past. This also means you're free to not release anything for a while if you want to take a vacation - there is no expectation that you must keep delivering new features.

While you could try to maximize the amount of money you earn, consider that for any amount of money you earn less than the maximum, it's value to your customers. Don't maximize, satisfice: figure out what is enough for yourself and be happy once you achieve that.