I am running 10x12 RMA 1155 plates for work (LEO) and also use them for working out. However, my body build leads me to have issues finding a plate carrier that fits well. Im 5'9" 170 and have a Banshee now. This carrier works great with my work gear on but is pretty loose in the shoulders without my soft armor on.

I am considering keeping the Banshee in my patrol car and swapping plates into a smaller carrier to workout in. Ive looked at Blueforce gear and 5.11s so far locally and they seem like they may work. Any recommendations? Also, I am running, and plan to only run, front and rear plates for this setup.

If you're just using the spare for workouts i'd buy a replica JPC. Cheap and minimalistic.

The fit is due to your shoulder set up for your soft armor.  Another carrier, same size, but set up for no soft armor, with the add on shoulder pads, would likely solve your fit issue. But it will never fit perfect due to being single curved. The back plate especially will want to pop up at the top or bottom off your torso.

Being a non standard, non SAPI shape, a JPC, or even a fake JPC, is a poor choice.