I tried it once more and could proceed! Interesting mechanics and soothing puzzle game, I felt so relaxed while figuring stuff out, haha.
If you want a tip about the lagging in the web build: The godot web builds lag a lot when loading gpu particles for the first time. So every time a new gpu particle is instantiated, the lag will occur. So what I do is I have a scene with all of my gpu particles and I make the scene completely black. When the game first starts, this scene loads for ~1 second, plays all gpu particles in the dark, then the actual game starts.
When playing no lags will appear from gpu particles :)