unraid uses docker, and docker is everwhere the same.
-c is is the same like --cpu-shares
It works liek that
YOUR CPU POWER / 1024 = 1 C
So if you have 2 dockers with 512 parts, it means each docker uses max 50% of all CPU power. (if its fully utilized, if not then all programs will use 100% - thats the great thing about this)
If plex is most imporatnt for you, you dont set any c, which means default 1024 (100%).
All other dockers u set to 2 4 or like you want. Just try it.
I have 2 or 4 for all dockers and only nzbget 128, so i get fullspeed even when 100% cpu load.
I cant believe you have really problems with your cpu, since you have double passmark score then my X years old cpu... and before i even had a much worser