If you have 1000Mbps Download and 1000Mbps Upload, no need to use any sort of bandwith control. Other devices affect latency is not much ( only +- 10ms difference )
Have you ever tried with no QOS ? I also tried my 800/200Mbps with FlexQOS or other type of QOS and the best result I get is with No QOS.. ( high bandwith, low latency spiking when downloading )..
If you use wifi, you must ensure that your kids connect to different wireless band..
I think the issues are from PS5 itself.. There are many people complaining about this too..
PS5 1GIG Internet Speed Test - YouTube
PS5 Slow Wifi Speeds : PS5 (reddit.com)
Some of user follow this steps.. The rest you need to research by yourself..
-Alright so this worked for me for some...