Smart QOS is going to help you if you run into an issue where one person downloading/uploading a large file brings everyone else to a halt or if you’re overloading the buffer in the modem.
IDS is to detect bad actors trying to break into your network. So these are kind of separate concerns from your Plex server.
See how it works with a vanilla config and then troubleshoot from there. If you find that QOS is necessary and it’s limiting the bandwidth that you’re paying for, it would be worth looking into using pfsense on the edge and just letting the Unifi manage the LAN.
If you do get to the point where you want to add a pfsense gateway for cheap, you should look into getting a used thin client. You could even get 2 and set them up to fail over. They aren’t expensive.