I have another idea for making a openworld digging ground videogame and keeping it at

Make the faraway ground be composed cubes and rectangles to limit the draw calls.You make any tunnel or hill out of that. And the ground faraway is set to no collision.As you dig python would add cubes that would extend over exposed cubes.Making rectangles whose positions and sizes which are stored in python.

I thoroughly reccomend you try making some of your ideas.

I believe this is the best one.And i will try to impliment it.