I wanted to start this post to get some feedback on my plan before I actually do the build, so the pictures attached are of the current build before it becomes "Project Dark Water". Pictures of the build will come later (edit: skip to post 21 to see the new build up and running).
 
Old system parts list:
bequiet! Dark Base Pro 900
EVGA X99 Micro2
Intel i7 5820k @4.2GHz
EVGA 2080 Super XC Hybrid
G.Skill 32GB (8GBx4) Quad-Channel 3200MHz, 15-15-15-35 DDR4
EVGA 1000W G3
EVGA CLC-280 (with additional silent-wings 3 fans on back for push/pull)
2 old spinning drives with backup storage, 1 NVME drive in a PCI-e adapter card (this board doesn't have m.2 for storage drives), 1 sata SSD, 2 optical drives (fairly ancient...very infrequently used...going away)



 
Images don't have new mouse and keyboard (EVGA X17 and Z20).
 
So, Here's what I'm looking at for post-water-logged-build:
Dark Base 900 Pro (hence "dark water")
EVGA z590 FTW
Intel i9 11900k (OC's TBD)
G.Skill 32GB 4000MHz 16-19-19-39 @1.4V DDR4 F4-4000C16D-32GTZR (stock config. not sure where we'll end up)
EVGA 2080 Super XC Hybrid (this stays for this year. 3080ti Hybrid would be the only thing that may have a chance to convince me to upgrade this horrible cycle)
EVGA 1000W G3
Keeping HD's, scrapping DVD drives.
 
OK, here's the fun part that I'm looking for some feedback on, WATER LOOP:
Optimus Foundation CPU Block (acrylic + black aluminum)
Optimus Absolute Reservoir (8.5", satin black, 3 port plugs on top, getting a blemished one for the discount)
EKWB D5 Xylem PWM pump (sleeved)
EK-Coolstream CE 420mm Radiator (I have a whole bunch of silent-wings 3 PWM fans. Going to put 3 to push and depending on case limitations above it, probably 2 to pull. I don't think I can fit a 3rd fan over it because my case has a phone charger do-dad up there that is probably in the way)
EK matte black tubing 16/10mm (I have used this at work, I love the dark industrial look and think it'll be sweet in this case)
EK Quantum Torque compression fittings
EK Quantum Torque Splitter T
EK Quantum Torque Drain Valve
EK Y-splitter 3-fan PWM cable
EK Y-splitter 2-fan PWM cable 
      (I think the motherboard has 2 "CPU fan" headers. I'm hoping I can run 3 fans off one, and 2 fans off the other, pump plugged in somewhere else)
EK CryoFuel Clear (I don't want to mess with colorants falling out over time)
 
I have an EKWB filling bottle and unlimited supply of DI water at work, so I may borrow that to fill, leak-test, and flush the system before I put the EK fluid in. I am planning on putting the T-splitter near the bottom of the loop with the drain valve out the side of that. I got an extra fitting so I can run some tube down from there when I want to drain it. I don't think I'll need anything other than straight fittings, but if there's some problem, I think I may have some black Koolance 90° fittings just collecting dust at work that were extras from an old project and may just be scrapped that I can borrow until I get some matching ones. Looking at the manual page about pump mounting (attached), it looks like the bracket mounts to a drive bay. I'm thinking I take the two I have and move one of them under the shroud with the second one just on top of the shroud, mounting the pump on top of that. Without the Optical drives there (that whole bracket comes out), I think there'll be plenty of room.

as long as the reservoir is mounted higher than the pump ... so pump never runs dry .. do what you want
 
You just need a gravity feed to keep the pump wet with coolant
 
If your willing to drill some holes in the case ... mount the pump anywhere you like
 
I find it best to have the parts on site to do the visual testing, where it looks best & plumbing will not be a pain
 
That should be plenty of radiator for just the CPU

What a great looking mod. 

Cool GTX
as long as the reservoir is mounted higher than the pump ... so pump never runs dry .. do what you want
 
You just need a gravity feed to keep the pump wet with coolant
 
If your willing to drill some holes in the case ... mount the pump anywhere you like
 
I find it best to have the parts on site to do the visual testing, where it looks best & plumbing will not be a pain
 
That should be plenty of radiator for just the CPU

Thanks!
Yes, the pump mounts into the bottom of the reservoir, which will be at the bottom of the loop (but with res over it, I guess I could mount it anywhere...but it's at the bottom)
 
The PSU shroud for this case was a late addition as it wasn't available when I bought the case and I only bought and installed it a few months ago. It was...way more complicated than expected because there's almost zero access to two of the screws they want you to put in and the LED strip I had double-sided taped in place was sort of in the way. That's an easy fix, but I just didn't want to deal with it at least until I have the whole thing torn apart for this upgrade. Anyway, There are a bunch of nice removeable panels on the psu shroud, so if the hard-drive cage plan doesn't work, I can always put a couple holes in one of those panels and mount the reservoir there. It doens't come with brackets, but there are two holes in the back of the pump-top and four holes in the bottom. One way or another, it will at least someone line up with either the pump-mount bracket that came with the case, which will then be mounted...somewhere...or directly into two new holes that will magically arrive lol. I will know more when I have everything in and take out the DVD drives to do more fit-checks.

glanced at your attached jpg - top & middle pictures looked like a stand alone pump
 
OK, your reservoir has a pump head built in 
 
 

Nice and clean.  

Parts are arriving!
 
Well, the CPU, memory, thermal paste, and some new fans have been here for a few weeks now:

 
The EKWB parts have all arrived too:

 
I did notice something a little odd and unexpected about the pump, in that the impeller top plastic is all scuffed up, straight from the factory. I did some googling and even asked Performance-PCs about it (I ordered that separately after finding out the reservoir I ordered didn't come with a pump...woops) and they said it was normal. I don't think the torn cardboard in the box actually hurt anything and the rest of the pump looks flawless, so I'm thinking it's probably fine. I will definitely run a test with just the pump/res and a tube just to make sure there's no rattling or anything.

I heard Optimus was the best, I may want to go the route you did with the Optimus Absolute Reservoir B stock and a separate D5 pump. Let us know how it goes, excited to see the final result. 

CPU block came in:

The CPU block actually came with kingpin kpx. Now I have to decide between the pk-3 and the kpx. Anybody have any recommendations? Reviews seem pretty split between the two. I think the pk-3 needs to be spread across the ihs because it isn't as fluid and won't spread so well itself, which leads to some of the review differences. Even then, I'm not sure if it makes any difference or if there's even a winner between them lol. I'd like to do one application and not touch it for a long time, if that impacts any recommendations. Thanks.