First, set your min volcanism to 100. Next, in the parameters themselves (not in-game), crank your max volcanoes to something absurd, like 8000, which will make about half your world tiles have a volcano on them. Next, to increase the number of land spaces, turn min elevation to 99. It won't have much effect on what you're looking for, and oceans are sort of bad to embark on anyways. In addition, I'd tinker with elevation. Give heavy weight to mid-low elevations (weighted ranges, and make sure to set as 2x2 or it won't work); volcanoes like to make mountains, and mountains aren't nice to embark on and make rivers harder to get nearby. Next, set elevation variance to something very low, like 20, but still require a mountain peak or two or three so dwarves can exist. The peak will force high elevations to briefly exist, it's funky like that.
And finally, as a little suggestion, crank those semimegabeasts way up. Semis don't mess with civilizations like megas and titans do, so you can have basically as many as you want. I've gotten over 1000 in a 33x33 with little issues other than a slightly longer wait before world gen actually starts.
For actually finding the spot, try the DFhack command embark-assistant. It lets you search specifically for a lot of things, and will even let you minimize the evil effects that the biome has, supposing you care about it.