Unity could potentially get squeezed between Godot in the low-end and Unreal in the high-end. I've played around with Godot, and for small, performance-insensitive games, like a Candy Crush clone or indie puzzle platformer, it is a nice, license-unencumbered option.
I heard a lot of good things about Godot Engine: https://godotengine.org/
Someone also mentioned here Grid Engine (which I never heard of): https://www.planimeter.org/grid-sdk/