I like all that (except the ballistae).![]()
That all sounds good.
How about giving it a move of 20 ft., with Run and Fleet of Foot as bonus feats? We could swap the Run for one of its feats, like Combat Expertise, but can't give it Run 'cause its Dex is too low.
Make it so, dropping Combat Expertise. That feat seems incredibly odd for what is essentially a steamroller!
Do these have any shapechanging powers at all?
Did any of the other mimics have spell resistance? If not, I'd drop it. Ditto for the energy resistances. I picture these things as big mimics that take the shape of a juggernaut, not living, amorphous juggernauts. YMMV.
Boosting fast healing seems fine.
I think I'd agree with Shade here.
Do these have any shapechanging powers at all?