Planetaris Unbounded: Arig adds another tier of transport belt, causing an error on load regarding next_upgrade_target not having the same fast_replaceable_group ("planetaris-hyper-underground-belt")

I took a look through the code, and I think the current implementation is actually very rudimentary and prone to breaking. Firstly, I feel like this sort of change should probably be done during final-fixes instead of updates (though I might be wrong on that front). And secondly, you could probably do these changes programmatically, by running through every splitter/underground belt in the game, and then changing their fast_replaceable_group replacing several if/else blocks with a simple for loop.

It's been 11 months since the last update to this mod, and 8 months since the last update to any of your mods, I'm going to assume you aren't coming back to Factorio modding anytime soon, and I'll just make my own implementation instead, because I really need this feature to work.

EDIT: I threw this up in about 20 minutes, Better Belt Legacy