I figured out the solution.
So, to re-state the problem and it's cause: When sending a node out to a broadcast monitor via Blackmagic hardware (by clicking number key 3), the output looks dark and over-saturated when the project or timeline's color science is set to "DaVinci YRBG Color Managed". The reason is because Fusion is automatically converting all properly interpreted footage into a linear gamma, which is optimal for compositing math.
So... here's the solution.
Simply add a "Color Space Transform" node into your composite with the output Color Space AND Gamma BOTH set to Rec.709 (or whatever your monitor is set to).
Then connect your final node, or any node you'd like to preview on the monitor to that Color Space Transform node... and set it to broadcast (click the number 3 key).
Done. Works perfectly and has no impact on output.

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