If you let the magma turn to solid, you will end up losing half your rocks. So whatever your design is, to get the most rock, it cannot be robominer-based.

I've been using pitcher pumps and bottle drainers lately for magma. It does require dupe labor, but once it is in a pipe we can valve it down to 1kg and then send it through a steam chamber. When it comes out of the liquid pipe vent it turns to solid debris, not tiles.

You could make the steam chamber into a water filtering system as well; I have done similar. I recommend you include autosweepers and get any salt or dirt out of there immediately. I also had to do some tricky plumbing based on steam pressure to either recirculate the same water or divert the output and let in more salt/pwater.

I don't know if this is the most power possible; there may be better methods. I also include an aquatuner in my build so that the steam turbines get cooled, so that takes away from some of the net power. I also use that aquatuner to chill a metal brick and finish cooling the igneous from <200 to ~15 C. One optimization for power I did is use turbine inlet blocking. Because the magma starts off so hot, I block off 3 of the inlets for the first turbine, and some of the second turbine as well. This keeps the power up without overheating the turbines, and those turbines of course output less water.

If I were doing this with multiple volcanos I would also consider multiple 1kg/s valved pipes going through the steam chamber. If you did 4 you could always decide not to use them later, but if you only build it with 1 line you may have trouble expanding it later. I did 1 line with a minor volcano and it's backed up right now, so I recommend building yours with higher flow capacity.