You might already be aware of this trick, But you can build your own output monitor setup using Advanced Output and Spout (NDI will use a TON of resources, so use Spout!).
Not sure what look you are going for, If it's a side by side, a Top to Bottom, Or just a giant square, but you can essentially build a custom layout in Advanced Output the way you want it to appear in the UI.
For my example, I just did a side by side view. But again, just change the concept to what you're looking for.
If you want it to be a top to bottom, just re-arrange the way you configure your Advanced Output.
In Advanced Output, Create a screen to act as your Visualizer. For me, I wanted a Composition view, A Preview, And a Layer 1, 2, & 3 View. So that's a total of 5 different views.
So I made a 1920*5 (9,600) Wide composition and left the height at 1080.
I made 5 slices and gave the inputs to match their function (i.e. layer 1, layer 2, preview, etc.)
I aligned the output of that screen to show them in the order I wanted, leaving the inputs the same (centered in the comp)
Then I made the screen output to Spout.
I then made another screen. This will be my actual output to the LED wall.
I made a 1920x1080 slice and left the input and output alone.
I can assign this screen to a HDMI port on my machine.
I set the input of that slice to "Group 1"
I changed my composition to match the visualizers sizing.
I turned off Auto Fit/Fill on each layer.
I added Layers 1-3 into a group and left it named as Group 1.
I added 2 more layers on top of Group 1.
The layer directly above it has a black solid color, to act as a buffer. (If you don't have this, Group 1 will overlay on top of your visualizer, as that is where the video's are being played back by their default X and Y values. This black solid color will blackout anything from Group 1)
And on the op layer I added the Spout source that we are outputting from Advanced Output.
Now I have a customized output window.
Even with the crazy large composition size, my machine is still running at 60fps when using Spout, as it's pulling straight from the memory buffer. If you use NDI, you're going to get a TON of extra processing involved and it will bump you down to about 20fps.
If you really wanted to be cool, you could add an extra 10-20 pixel in between each slice in Advanced output, and maybe even 40 pixels on top.
From here, you can change your buffer blackout color to Red, and add a few Text block FX to it and label your windows and this would also provide you with red bar's in between each window.
Another thing, If you build your custom layout to be somewhat close to a 16:9 or 4:3 setup. Rather than sending it to your UI. You could connect a second monitor up to your machine and output the visualizer screen in Advanced Output to that monitor, and keep you UI without one, giving more room for clips, and fx.
I think having a preview, independent from the output monitor would be very cool. Until then, maybe try this or some of the other suggestions above?
It's more work right now, but once you get it set, just save everything as a template and go from there for future shows
Skip to 3:10 for the final product: