I believe itch has an option for you to require certain purchase prices to receive certain versions of a product. With other products, I've seen options like $X gets you the base version, $(X+Y) gets you the bonus version, and $(X+Y+Z) gets you the premium version; or, the most recent but 1 version is free, but to get the most recent version you have to have paid $X.
The suggestion, therefore, is to set the prices for how soon someone can get the most recent release (for example, $25 to get it as soon as it's released, $15 to get it after 2 weeks, $5 to get it after a month). Then, you just move the new release down through the "tiers" (yes, like with Patreon) as it gets old enough to open up access. This would let people buy in at the level of "freshness" they want easily. Right now, if I'd be willing to buy in after a month, I have to be sure I look in at the right time to pay (or so I assume), and have no access to an older release once a newer one exists. Among other things, pricing is clearer and more visible; even though I know what you're doing, constant price changes make things feel unstable.