If there aren’t any more demo updates in the future then I’m afraid this may harm your business more than anything. Since it’s still a prototype/playtest that will take a long time to complete, it won’t gain much traction. I recommend rather than a monthly subscription, try tips instead where people can pay you if they want to support the in development process. you’ll make $$ when the game fully releases but a monthly subscription to see small updates? It’s iffy. Probably won’t do well in the long run. Try ko-fi.
(genuine advice from a business/marketing pov)