Thanks to @krayzkrok’s tips, I found a few fissure asteroids, and before I knew it, was returning to San Tu with 19 million credits’ worth of various pretty rocks from the next system over, some of which sold for north of 800,000 per ton.
I went mining in a rocky ring rather than an icy ring, so the asteroids look a little different, but hitting the most distant planet’s ring system seems to have made a big difference. I came across three crackable asteroids over about 40 minutes.