You don’t need a chest rig
I wanted a chest rig to take to the range, but I’m cheap, and the DPM Load Bearing Webbing allows you to carry a butt load of magazines for a fraction of the price. In my opinion, it’s also more comfortable. Some assembly required, and expect to fiddle with it until you get everything set up how you like it. Also, if you are carrying all the weight on the shoulder straps, you are using it wrong. I set up mine like a backpack, with the hip belt above my hip bone and pants belt, carrying 90% of the load. Your shoulder straps should keep everything from flopping around and stabilized, but the hip belt should carry the bulk of the weight. If you end up with the normal small belt, as long as it uses the square plastic Fastex SR buckles, you can purchase an ALICE belt extender to give yourself a few more inches. (That’s what she said.) I prefer PMAG to metal and the pouches carry 8 comfortably. I’m confident if you were using aluminum magazines, you could indeed carry 12. One of the utility pouches comfortably carries a GI canteen. The other is useful for first aid and snacks, or as an admin pouch, with plenty of room. Your mileage may vary, but I have used this with and without a plate carrier, with no issues.