I've driven a DEX210 in MM4 on clay - not RHR, but I think good enough to get a nice impression of a mid DEX on clay. It was better than my TRF201 (that admittedly had some setup issues), better than a +8mm B4 I drove on the same track (which was nice but you're always slightly understeering or oversteering, there wasn't a neutral point it seemed), and in fact all mid-motor cars I tested that weekend just worked MUCH better imo (X6 Sq, Mid-TLR22 and MM4-DEX210).
I think you should give MM4 a shot - For me it felt magical: The car was lightfooted yet stable, and the rear bite was actually better than on my rear-motor 2WD. It had a high amount of steering yet didn't seem to unnerve the rear end with that - top stuff! The only remark I could make was that if it did lose rear grip it was a bit snappy, but admittedly still less snappy than on my own car!
I know the guy that drives the MM4 210 uses no weight up front, a small spur/pinion combo to push to motor to the rear, reduced steering on the car in general and I recall also drives little anti-squat on the rear. I also know he doesn't use weights behind the gearbox.
Edit: I'd like to add to this: I heard a statement about another 2WD - just food for thought really
The statement was that the RM seemed to offer more raw pace, but was harder to drive at it's top pace. The lack of consistency resulted in several considerably slower laps each heat. The MM was about 0.2-ish seconds slower each lap, but was more easy to get around, making it possible to hit the best lap times through the whole heat.
All I'm saying is it can't hurt to try it out, and from what I've seen and my (short) experience has told me, MM works surprisingly well. Being that probably no-one drives a Durango in RM in the UK, why not use the extensive MM knowledge on the car for low bite?