The CSV-7300 is not a splitter. It is a DisplayPort 1.4 MST Hub. It allows connecting multiple displays to a single DisplayPort output. The GPU must do all the work of creating the streams that are to be output. Therefore, the only reason you would use the CSV-7300 is if your GPU doesn’t have enough outputs, which you say is true in your case.
An MST Hub can duplicate a single stream to multiple outputs. But since the streams are the same, they must have the same resolution and refresh rate. This feature is not what you want.
So the only way you can do what you want to do is use mirroring from the GPU to the display and capture card through the MST Hub. It should work but it uses additional resources of the GPU so there might be a slight performance difference.
I’m not sure how well mirroring works in Windows. In macOS, I can mirror to two 4K displays one at 60Hz and the other at 95Hz (not using MST Hub since macOS doesn’t support MST Hub).
Before you enable mirroring, try to get one display set to 120Hz and a second one set to 60Hz from the MST Hub. You need to get two separate displays working before you can try to mirror them. You might need to use CRU to create a custom resolution if the resolution you want is not available.
I guess first thing you should verify is that a display connected to the DisplayPort output is actually connected directly to the Nvidia GPU instead of the integrated Intel GPU. Check Device Manager and view by Connection Type.
Your laptop has Thunderbolt ports that support two displays but they may be connected to Intel GPU? Connect a display with a USB-C to DisplayPort adapter or cable and check Device Manager. A smart laptop like a MacBook Pro would connect Thunderbolt displays and internal display to the discrete GPU instead of the iGPU.
The iGPU in a Comet Lake CPU is limited to DisplayPort 1.2 so you’ll want to make sure it’s not being used for display output.
Normally an Nvidia GPU can output a max of 4 displays. Your laptop has 5 outputs though so I’m not sure what’s going on (two Thunderbolt, HDMI, DisplayPort, internal display). If you don’t have 5 displays to test, you can get inexpensive dummy adapters that fake a display.
An Intel GPU can connect 3 displays. I don’t know if there exist an laptops that will let you connect displays to both GPUs at the same time.