I love your style! The demo is very enjoyable and has a lot of potential.
The core gameplay loop is solid, but I find the peace a bit slow. Maybe you can try to make the first upgrades cheaper or slice them in smaller upgrades that cost less. It's depends if you want to make the game more idle or more action.
I like the story, is simple and engaging. I'm genuinely curious to know more about it. I love how dialogues are consistent with the art style and the game in general, in my opinion that's something you got it right.