Desire to use my Linux NAS to replace my Hikvision NVR

Currently I have:

Atom (Braswell) NAS with 8 GB of Memory and 2 NICs

6x Hikvision Cameras (DS-2CD2032-I 3MP IP Camera)

1x Hikvision NVR (DS-7608NI-SE/8P) with a 4 TB HDD

The individual cameras currently do all of the heavy lifting in terms of looking for motion events. The cameras each process their own video and flag events that meet the criteria for recording on the NVR. Apart from recording flagged video the NVR also displays through HDMI to a computer monitor to display the combined video from the 6 channels.

What I'd like to do is:

1) Get rid of the NVR

2) Put the 4 TB HDD into the NAS as another drive.

3) Power the cameras with a POE switch which I'd need to buy (NVR is currently powering the cameras). Attach this switch to the spare NIC.

4) Have the 6 cameras use the HDD in the NAS to record their video (this should be doable as its an existing option in the camera)

5) Find some way of using the NAS to display out of its HDMI port the combined 6 channels for us to view through the computer monitor.

I think each of the items 1-4 are fairly straight forward. However, I'm not sure what I could use to display the combined camera images to the monitor. I'd like it to be something that auto-starts every time the NAS starts up. It would be a big pain to need to manually start this every time the NAS starts.

Is this a foolish plan or does this look like a doable configuration? Any ideas on what software could be used to display the combined video to the monitor?

Thanks for any advice!

Since you already got the NVR and the setup, why are you considering ditching the NVR ?