Hello, not trying to be rude, I'm sure this is a lot of work; seriously, game development is hell on earth, but I'm certain a lot of people would like to pay only once. I get it if that isn't viable for you.
dude a lot of games do this when it's in early access some use Patreon or subscriberstar plus this game needs to be crowd funded because the game isn't out yet so if everyone only bought it once then there wouldn't be that much development