This is such a vibe - as far as idle games go it's definitely one of the idle games of all time, and I don't mean that negatively! It's very simple, which is good, especially for a prototype.
If I were to have a single complaint, it's that everything moves at a different speed relative to the player's "run" speed. The ground is moving really slow, which doesn't lend to the illusion that the hero is running. I also wish I had more to buy in the shop, but it seems for now at least, the goal is your coins per second.
I love the music in this too! Dreamy, which goes well with the background :D
Gamemaker Studio 2 is a good engine, I would also recommend Godot if you want a free option.