Hi metalx! Sorry for the long response. I would love it if we could work together to further develop the game if you wanted.
STC has some long-running optimisation issues. Ever since I developed the level editor the game has become sluggish, especially on mobile devices.
I would also like to port the game to Godot 4, but I haven't had time. Doing this would allow me to fix a number of hacky workarounds I had to implement for the game to work properly, like the object tilemaps and water tilemaps.
As for Milestone 2 features, I never looked into that but it would be fun to add some. Which ones were you thinking?