well thats an epic fail. so which planet type tends to have the most "randomly generated" number of asteroids? im trying to make an awsome map and im trying to make a heavily contested resource rich area thats hard to access.
Asteroids, Terrans, and Deserts have 2-3 asteroids, volcanics and ice planets have 2-4. Asteroid belts, space junk, gas giants, and plasma storms have 0-3. Other gravity wells never have resources.
The best way to add resources is to fill out the area with a few extra asteroids. Roids are cheaper to colonize than full-sized planets, give negligable tax income, but grant the same resource income as full-sized planets. Other than this, you really won't find a big difference between the four colonizable planets. You could also try filling out the area with uncolonizable gravity wells, but place too many and this unfairly advantages Vasari, who can get to them earlier with their speedy scouts.
I have the same issue as you when I'm making maps; there is just too little fine control. For my purposes, it's creating balance between players for multiplayer maps, for your purposes it's controlling the resource density in specific regions. Either way, we just don't have that level of control with galaxyforge.
I know there are certain "custom planets" that are put into mods and whatnot
I'm certain you could do something with a mod, but this would be dependent on running your mod to use the map. If you want a stand-alone map, this is not feasible.