the concept sees cool. The game need optimization, as you said.
- Disabling shadows option
- Compress texture & audio
- Hide polygons/models when the player is not looking
- use a fog like minecraft for hide things far from player
Special tip: Use "var :=" on variables & constants on Godot engine. In Gdscript the variables can change type, when you use "var :=" instead of "var =" you optimize the definition for the game engine. So Godot don't need to discover and define it.
btw: just suggestions, thanks for reading.