I'm not sure if space platforms can really function without any asteroids whatsoever? Is this an attempt to reduce the amount of lag in the game? Or did you mean to reduce the amount of dangerous asteroids (since there's no other reason to completely eliminate the asteroid chunks).
The minimum percentage isn't anything hardcoded, it's just what seemed to be minimum viable amount for space platforms to use for fuel to propel themselves.
I'm honestly surprised that there isn't a mod on the portal to completely disable all asteroids, it actually wouldn't be that difficult (instead of multiplying the spawn probability, could set to zero, or just remove the field entirely). I'm not sure how viable that would be.. you would have to use rockets to mail in all the resources you need to propel the platform to the next destination, or else just let it drift after it runs out of fuel
EDIT: Hmm, looking further into this concept, it would be basically impossible to create space science, since there's no way to create carbon or ice in vanilla (you don't get the alternate recipes until you reach the other planets), which locks out thruster and planet discovery techs. Would need to create some kind of alternative in order to make the concept work
All that being said... I think this might be possible with Muluna? Since it moves the thruster earlier in the tech tree, allowing the player to reach Muluna, which provides access to asteroid mining and processing using resources from the moon instead of directly from space. Here's a concept test that no longer requires Muluna, uploaded via Google Drive