Do you see anything different, in terms of video quality, if only one of the mini PCs is running Studio Monitor? And what does the quality look like if you run Studio Monitor on the vMix PC? I'd suggest running each one individually (i.e. just one mini-PC at a time) to check you get a decent picture, and then try adding more. If the quality drops when you're running more PCs then check the network data rate being sent from the vMix PC.

NDI is definitely not the ideal choice for distribution to multiple screens.

Why?
- it requires very powerful, thus expensive, computers for each monitor
- it requires a lot of bandwith which might saturate very quickly a basic natwork infrastructure
- it's not really easier to manage remotely than other protocols

All this makes it the most expensive and the most demanding protocol to use with no real benefit.

What solutions then?
Something like Multicast UDP is highly recommended for the exact use case you described (unless you absolutely need low latency or real time):
- requires a simple FFMPEG script to generate the feed out of vMix (or a cheap external encoder, less than $200 if you prefer)
- will run perfectly on a $35 Raspberry Pi connected to the monitor
- will only use a total of 10/12 Mbits for bandwidth in order to feed up to 250 screens, this will work on basic networks even with a crappy Wifi with an excellent image and sound quality
- automatic play can be very easily scripted on the player side so that no manual intervention is required to start playing

If you need low latency a simple pack of HDMI to CAT5 converters like this one https://www.amazon.com/M...rk-HSV373/dp/B00T76THZ2/ is much better
With 1 sender unit, you can feed up to 253 receivers. No computers needed at all.

Summary
Multicast UDP is at least 10 times less expensive, delivers an equivalent image quality as NDI using at least 10 times less bandwidth thus it's 100 times easier to deploy and run.
HDMI to CAT5 solution costs a little more but it's all automatic, just plug and play!

NDI is for production, not distribution...

My 2 cts
Guillaume