Even if the displays don’t have DSC (they are DisplayPort 1.2), the MST hub should be able to take a DSC input and decompress it for the DisplayPort 1.2 displays.
DP 1.2 is 17.28 Gbps.
DP 1.4 is 25.92 Gbps.
Adding an MST Hub does not increase the number of displays a GPU can support. NVIDIA GPUs are usually limited to 4 displays. AMD GPUs are usually limited to 6 displays.
For the refresh rate calculator, the Resolution is supposed to be width x height, not pixels x displays. To calculate what you want, you have to determine the max data rate for a display (DisplayPort 1.4 with DSC = 25.92 Gbps x 2 compression factor / 3 displays = 17.28 Gbps) which in this case is similar to DisplayPort 1.2. So use the refresh rate calculator to get the max refresh rate for 17.28 Gbps = 174.2 Hz which is very close to what you calculated.
The delay for the CSV 7300 should be small because it doesn’t need an entire frame to do DSC decompression.