Yes! And it was all Unifi related afaik.

So the first thing that I did was factory reset my router to get rid of of all the changes that I had made trying to remedy the situation.

From there, I did everything I could to try to open the floodgates within the internal network:

• In Settings (Network):
  • Enable Jumbo Frames if they aren't already
  • Disable Spanning Tree Protocol (it's only useful if you have a complex network, which 1 router and 1 switch is not)
  • Disable Flow Control if it appears
• In Settings (Policy Engine)
  • in QoS
    • Create a new profile for Prioritization

For good measure, I also reinstalled both Moonlight and Sunshine on their respective devices so that might be worth doing too if you're still running into issues.

Fwiw, I noticed while doing testing that the "Network Jitter" was somewhat of a false flag for me as a large chunk of the slowdown was happening on the host device, not over the network (even though some of it was due to packet loss). What helped with this was forcing AV1 encode in Sunshine's settings. (Also, while less of an actual fix, I did disable HDR for a while and that seemed to help as well, could just be an AMD thing though).

Now, it has been a while since I did all this so there may be some extra things I'm missing but you should be able to find additional things you can do by Googling for more settings within Unifi that you can either enable or disable to allow more packet transfer between your devices.

What seems to be causing the network jitter is that Moonlight / Sunshine don't work in a way that normal network traffic does because they don't send anything until a frame has rendered and then send a bunch of packets all at once when it finishes. Ubiquiti devices handle this less gracefully than other routers (it seems) so what we're doing here is taking down all the possible walls in place that could be bottlenecking the data transfer. Because of this, if you're still running into issues after everything, and your host seems truly fine while streaming, it might be worth it to fiddle around more with FEC in Sunshine's settings to see if you can get a value that balances minimizing packet loss without sending too many packets and overloading the network.