Well, for starters, are you monitoring your hardware to know where the bottleneck(s) is/are?

Quote Originally Posted by Chumbucket View Post
I actually hadn't thought of splitting the load between multiple computers though I have done so in years past. You've touched on many of my concerns as I think my real question is, with the new graphics requirements what gear will give a 5 boxer what type of experience in the current graphics environment?
The hardware in your machine is over 5 years old, but any current mid-to-high tier hardware is going to allow you to multibox with little to no issue. My latest video shows me multiboxing in several zones of BFA with practically no issues outside of Dazar'alor, where 3GB of VRAM was the limiting factor, and that was on hardware that is older than yours. However, any of my other gameplay videos from the past years show me playing at ~4K @ 60 FPS, which is 4x the resolution of 1080p, on a 6950X that is almost three years old and a 1080 Ti that is a year and a half old, and most mid-to-high tier hardware is better than what I currently have.

Quote Originally Posted by Chumbucket View Post
My current system baffles me to some degree as I'm able to play a single account at about 156fps with level 7 settings in high populated areas. When I play 5 accounts however, I can generally play in the world at 60fps on my main and 15fps with the alts with all accounts set to 1 (DX11 Legacy) and using Slot Swap macros to boost a few things slightly, like distance to 5 and environment to 3 and downgrading alts resolution to .3 of the main. (Everything else is 1). In Stormwind however, my fps drops very low at times and sometimes my computer just locks up completely.
It's not that baffling, though, since 3GB of VRAM is very limiting, and SLI doesn't increase the VRAM pool. So, while one game client is not limited by VRAM, five game clients are. If nVidia wasn't so stingy with VRAM back during the 700-series, then you'd have 6GB of VRAM (like the Titan Black did) and you'd, almost certainly, still be able to use those GPUs without issue, today. As for the machine locking up, then that's a hardware issue, and it likely has to do with your hardware not being stable at whatever clocks you're running.

However, not only can you drop each of those background game clients down another 20-25% on render scale, to save on even more performance, but you can "split the load" of the game clients across multiple GPUs if you have more than one display attached to your computer.

Quote Originally Posted by Chumbucket View Post
I was thinking of keeping my current CPU and just upgrading the GPU. Otherwise, I could build a system so I guess I can design one just for the alts or maybe go for a single computer option.
Again, you won't know how well your CPU is doing unless you're monitoring it while you play. If you upgrade your GPU, then your CPU has to do more work, essentially just creating another bottleneck if it isn't capable of performing. Personally, I don't think multi-computer setups are worth it unless you already have a second one lying around, especially when you're capable of just building a new machine to handle everything on one.

Quote Originally Posted by Chumbucket View Post
Though I would love to get the i9-9920x with compatible Mobo etc. it's just too rich for my blood. The 9900k seems like a reasonable option but I have seen no reports on how it performs multi boxing. It's probably fine. At least one person reported pretty decent performance with an i9-7920 and a 2080 ti. I can save a few bucks getting an earlier CPU but at what point does it not handle the job of boxing 5 toons in a mostly seamless way at 7ish settings with the 2080ti cpu?
It really doesn't require a 12-core CPU and a 2080 Ti to multibox five characters in WoW at 1920x1080 using high settings, and there's nothing wrong with a 9900K, which is, technically, as fast as my current CPU, and just a little slower than a 9900X.

Quote Originally Posted by Chumbucket View Post
I was thinking maybe the i7-8700k would be a good compromise? That one uses the Z370 mobo instead of the Z390 and saves a little money.
Personally, I don't feel that buying an old CPU is a good choice, unless you have a very good reason to do so (other than wanting to save a few dollars). Also, if you do want to make the move to 4K, then you're going to want more cores, and an 8700K is going to struggle before any other CPU mentioned in this post.

Finally, and I feel like I've said this, at least, ten times in just the last week alone, there are new CPUs coming out in June and July from both AMD and Intel. So, if you can wait another 6-8 weeks for more information to be released, then that'd probably be beneficial to your wallet.