From looking at the GRE web site, neither the 500 or 600 will decode Motorola Astro, which both MDICE and StarCom21 are.
As a side note, MDICE will never sound as crisp as StarCom21 on a scanner due to the nature of simulcast and the receiver likely 'hearing' more than one signal. Highland 821 is in a good position being about 4 miles from one of the simulcast sites and likely getting a pretty good signal.
Your system runs on the P25 standard. The PSR-500 and 600 will decode that type of system no matter if it's a single site or simulcast type system. Like I said in my earlier post I know several people who own PSR-500's and can successfully monitor the Hamilton County-Cincinnati Public Safety System here in Ohio. Which is the same type of system the MDICE is. And I'm not sure where your getting that they can't decode Motorola Astro?