I don't know much about moding at all, but I will spray my ideas anyway.Maybe you can make the small destroyable asteroids really large.
This would make it necessary to remove the large ones from the map.
Maybe the reason you can't make the\ large ones destroyable is that the program might be considering the large asteroids as an unalterable part of the terrain. Like a planet. I hear you can add planets, so maybe you can subtract asteroids and replace them with modified small asteroids that appear big.
Also, it seems to me that the small asteroids are generated around the ship and not there until a ship is close.
Modifying the small ones if possible would probably require changing several of there features to make them more real. Like mass so they don't bounce off the ship as easily, and they would have to blow up if hitting the ship too hard. Also, when you blow them up, they come back. It might be necessary to change there regeneration rate and density.
I just tried your idea, and it seems to work, at least a little bit. This time I experimented with the lava asteroids in the Schwefelwolke Lava Field (the diamond-rich field in Dresden).
First, I tried commenting out the 3db file referenced by the "DAsteroid_lava02" dynamic asteroid in asteroidarch.ini and replaced it with the 3db file for the "lava02_asteroid90" asteroid. The dynamic asteroids in the Schwefelwolke field were no longer little rocks, but HUGE rocks that crowded around the ship.
To fix the crowding problem, I changed the "placement_radius" in the rh05_schwefelwolke_lavafield.ini file's "DynamicAsteroids" field to something a bit farther. This made the enlarged dynamic asteroids appear farther away, no longer crowding around the ship.
I then restored DAsteroid_lava02 in asteroidarch.ini to its original state (correct 3db file reference) and created a new DynamicAsteroid, "DAsteroid_lava02_big", that used the big asteroid 3db. I went back into rh05_schwefelwolke_lavafield.ini and created a new DynamicAsteroids field for the new DAsteroid_lava02_big and restored the old DynamicAsteroids field to normal. And when I loaded the game and explored the field, both of the dynamic asteroids were there!
Finally, I went into rh05_schwefelwolke_lavafield.ini again and commented out the entire "Cube" section. I loaded the game, went into the field, and there were no more stationary, indestructible asteroids; just the dynamic asteroids, big and small.
In summary, if we can't make the big asteroids (defined as "Asteroid" in asteroidarch.ini) destructible, just get rid of them and create bigger DynamicAsteroid objects, and include as many of them as you want in an asteroid field by using multiple DynamicAsteroids entries in each asteroid field ini
There are two problems with all this, though; I still don't know how to change the hit_pts of dynamic asteroids, so shooting at the big ones has the feel of "popping balloons". Also, I haven't figured out how to make the bigger dynamic asteroids cough up more loot; all dynamic asteroids' loot appears to be controlled by the LootableZone field in the asteroid field ini files.
Here's what my new DAsteroid_lava02_big in asteroidarch.ini looks like:
[DynamicAsteroid
nickname = DAsteroid_lava02_big
DA_archetype = solar\asteroids\models\ast_lava02_90.3db <--huge lava asteroid 3db
material_library = solar\ast_lava.mat <--material/texture (no need to modify)
explosion_arch = dast_asteroid_explosion <--replace with a different (bigger) explosion effect if you want, I just didn't change mine
particle_effect = gf_dynamiclavaast <--lava particle effect?
And here's what the new DynamicAsteroids entry in rh05_schwefelwolke_lavafield.ini looks like:
[DynamicAsteroids
asteroid = DAsteroid_lava02_big <---new DynamicAsteroid ref
count = 120 <---higher count than normal (increase or decrease to control field density)
placement_radius = 1500.000000 <---farther placement radius
placement_offset = 90.000000
max_velocity = 100.000000 <---faster velocity
max_angular_velocity = 0.500000 <---slower turning
color_shift = 1, 0.500000, 0
You can create additional entries for dynamic asteroids in asteroidarch.ini and each asteroid field's ini file to add variety. Just remember to comment out the Cube fields to get rid of the indestructible rocks.