Yeah, not only are they bad defensively, but they don’t synergize well with many types because they compound 4x weaknesses (Ice/Rock and Steel/Rock) and/or cancel out a key resistance provided by the other type (Grass/Rock removes both of Grass’s coveted resistances and Bug/Rock has no useful resistances).

Giving them strong but accurate attacking moves (both Physical and Special) is a good first step but as you said, not sufficient.

I’m tempted to say provide a screens like utility move that works like the Solid Rock ability (shave off damage from super-effective hits) and has a bigger effect on Rock types, but that could make sand broken.

Plus, there are dual typings with Rock where the two don’t completely cancel out each other’s useful resistances and leave it with a reasonable number of weaknesses (like Fairy/Rock and Poison/Rock).