RSCは、私が作った惑星に飛行ルートを追加することになります。And that changes the length of the route.

I'm not entirely certain what you mean, but there is a setting in RSCompat (labeled experimental) that sets an override on all of the routes that stretches them out the further away from the sun they are, up to a certain point. You could try disabling that to prevent modification of route length (assuming you mean the route's distance value, in code)

I’d like the Asteroid Belt to stop creating new routes in other planet MODs as well. Also, I’d like the RSC to stop changing the length of the routes.

I think you have a few misconceptions:

• Asteroid Belt doesn't create any new routes, apart from a few connecting the vanilla and certain modded planets to the inner/outer Asteroid Belt locations
• Redrawn Space Connections doesn't modify the length (distance) of existing routes; if that is happening, it's because the setting that I mentioned is enabled in Redrawn Space Compatibility

Does this mean that the addition of a route from my planet to the asteroid belt is due to the RSC?

Yes; the whole point of Redrawn Space Connections is to remove/redraw all space connections so that there are no overlapping routes, and will add/remove routes as necessary to ensure that the "web" of planets has valid connections throughout.

Asteroid Belt only makes a few manual connections (before RSC runs), and doesn't make any other changes (which leads to many issues when combined with RSC; since neither mod makes any manual compatibility changes with regards to each other)

Redrawn Space Compatibility exists as a compatibility layer that runs after both RSC and Asteroid Belt, that simply removes any connections made by RSC that crosses the Asteroid Belt (and makes a few other compatibility fixes for certain other planets). This ensures that only connections to the inner/outer asteroid belt are retained, and forces players to pass through that connection point (as intended by Asteroid Belt)

I haven't installed your planet (because I don't really want to deal with 2.0 for this mod anymore) so I don't know what the situation is, but hopefully that should explain what is happening. (oh, and to reiterate, RSCompat is the only mod that could affect the actual distance of routes; if the relevant setting is enabled)