I just have all cores available to all toons, which is probably not optimal, but I haven't had any issues with performance at all on 5 characters, regardless of which zone I'm in or how crowded it is.
Graphics now is a 3090 FE, on a 1440 ultrawide @120 fps, with gsync. I haven't done ANY tuning yet - I plan on doing some slot swap macros that will turn down graphics settings for all the non-active characters, and turn 'em up for the active character when I switch toons.
Currently, foreground character is allowed to be 120 fps, background characters are 30 fps. Without doing ANY of that tuning I mention above, I'm able to run all slots at whatever an 8 out of 10 means to wow, and my active toon's FPS stays comfortably in the 80-90 FPS range which is quite nice. Once I tweak things, I'll be adding in a 4k display on the side for the followers.