NDI is not free, and does not use hardware accelerated compression, so its encoding/decoding use CPU. Not a whole lot, but some.
NDI load is constant when OBS is running, just like rendering load is. That load is the same whether OBS is encoding or not, so there's no "idle"
in this sense. NDI load doesn't increase when you start encoding, it's only dependent on frame content.
If using NDI on the machine is more load than encoding load, don't use it. A capture card would shift the load you want to move without incurring much extra. Using NDI is good for spreading rendering load (so that OBS scene rendering on the game machine is simple, and on the streaming machine is more complicated) or moving frames from a gaming machine without a strong encoder (like a pre-Turing or older Nvidia card, or an AMD card) to a streaming machine with a strong encoder (like a Turing Nvidia card, like the 1660.)
You have a 1660 and an i7 9700. Is there any reason you're not using a single PC setup? If this is the streaming PC, what is the gaming PC setup?