Split grounds: not normally advised, but is there an effective way to do it?

My 6-layer PCB runs two coreless DC motors - each switching on an H-bridge at up to 3A each. I also have an IMU that seems to be especially sensitive to electrical noise. In previous iterations of the board, everything shared a common ground plane, as general wisdom suggests, but despite my best attempts at physically separating noisy and quiet parts of the circuit and stitching around sensitive parts, the IMU reports garbage when the motors are running. The board is quite small, so more physical separation isn't an option. Note that the power supply for the IMU is very quiet. Also, I found that if I move the IMU chip to a separate board and connect to it with external wires, it works flawlessly.

I'd like to further isolate the high current, switching, noisy parts of the circuit as much as possible from the more sensitive parts of the circuit. My plan is to split the board into a noisy section and quiet section. In the noisy section, I'll have the noisy traces (both the power lines and the return lines to the ground point) on the outer layers of the board (layers 1 and 6). Underneath those (in layers 2 and 5), I plan to have a plane, or "shield", connected directly to ground and nothing else, that can absorb capacitive coupling. Underneath the shield, in layers 3 and 4, will be wires that cross between this noisy section of the board and the quiet section of the board.

Could this help?