Hi there!
At the top of each update I post a message that your purchase is for that specific update only.
Though in truth, I often add a follow-up update for free depending on the size of those two updates. For example, this post was for version 0.6, but I added 0.61 for free.
Itch doesn't let me do any sort of monthly fee or charge per update without creating an entirely new post, so I understand it can be a little confusing. However if I gave access to all future versions after any purchase, there would unfortunately no longer be any incentive to continue supporting me.
I'm really sorry about that.
But all of my free updates include everything the paid update has, it only comes a month later is all. I don't lock cheats or images or specific scenes behind a paywall.
Also FYI, each update comes out on the 3rd of each month, so I'll have a new update here in a few days.