Space Exploration adds "Deep Space Transport Belts," splitters, and underground belts, which are expensive end-game belts that operate at twice the speed of vanilla blue belts. There are 8 different colors of belts but the actual belt / splitter items are functionally equivalent / interchangeable. The underground belts however only link to their own color, so you can have multiple underground segments in the same linear space where they would otherwise conflict. Additionally the underground belt range is 17 tiles where blue belts are only 9.

As a proof of concept I manually shrunk an 8-8 balancer using the alternative colors.
Left: Traditional 8-8 balancer, not TU, color-coded for structure
Middle: Shortened version with matching colors
Right: re-colored with 2 colors.

I've forked the repo to explore the possibility of improving other balancer designs with new constraints for the additional colors. I'd love some guidance on where I might start, or if you foresee any large hurdles I might run into.

Finally, as a separate issue, I got an error when running an example from the readme. I ran calculate_optimal compute --threads 1 8 length and got ValueError: Wrong bound: 17 which I suspect is related to the DS belts. Thanks for any advice on this too!