I wouldn’t spend resources on making a demo unless you just wanted to anyway. Steam’s refund policy gives everyone a two-hour demo by default. Two hours of gameplay has always been enough for me to know if I want to keep a game or not.
Accordingly, I picked up a copy of your game. It looks like it’s my kind of game, but if I’m wrong, I’ll know well within the refund window.
I realize this doesn’t cover other game stores, but people can always test drive through Steam and purchase elsewhere.