Thanks for playing the game and the great feedback. This was my first time doing an RTS so I had to cut some corners, like relying on Godot native navigation and collision systems, to speed up development, which led to the problems you're mentioning. I might try to fix part of it after the jam :)
Regarding the goals and the mechanics, your feedback let's me know that my game needed a bit more of tutorials through the levels. The gist of it is that the green units are weaker and create crystals and the red units break crystals to make more units, but are stronger.
In the last level you need to find the right balance between creating crystals and sacrificing them to create the red stronger units. It is a hard level, as it is the last one in the game. If I end up having the time, I might make a play through the game. To be honest, sometimes while testing I won that level fairly easily, but sometimes I would lose it to a dumb mistake.