I don’t have a Shapeoko, so I can’t speak precisely to this, but I cut this stuff on my Nomad all the time, and have found that you can basically go as fast and as deep as you like with no ill effects. The only issue on the more powerful Shapeoko would be melting the styrene, so keep the RPMs as a low as possible, and keep the feed rate very high.
I suggest you give everything a quick wipe with a dryer sheet, it seems to help with the styrofoam dust clinging to everything.
For a nicer finish, slow down a bit. You can also sand the foamular very nicely. I find you don’t need to start below about 180 or even 220, and use a very light touch. You probably want to wear a mask.