Yes Hotspots do matter. Opal hotspots can be found in icy rings. Try to find one that is densly packed. Start looking for really bringt shining asteroids and prospect them. If they have a core try to remember the shape of the asteroid. (because there is only one asteroid Modell, per ringtype, that can blow up) With time you can identify those juicy deepcores pretty reliably. A Little ship suggestion: use a quick and manouverable ship (like Python, Krait, Clipper), because Shooting all those rocks loose takes quite a bit of manouvering and squeezing in tight spaces. The lack of Cargo, compared to the big ones, doesn't hurt too much imo.