I have reached an impass. Although I now have a pretty strong team of pokeymen (all in the Level 25 region), and can kill pretty much everything in a hit or two, I don’t actually know where I’m supposed to be going next. From the volcano, I went through a cave and bumped into Team Aqua again. Team Magma then showed up and chased after them. Apparently they’ve stolen a meteorite or something.
I suspect I might have to find someone who does stuff with meteorites, but I took a wrong turn and went down a “one-way” path which took me back to Rustboro City. Now I have to wander round most of the map again to figure out where I’m supposed to go next.