Just noticed that it has been exactly 1 year since I started this thread. There has been some scope creep, some diversions.
I have actually bought in a whole new set of AquaComputer stuff, rads, pump, leakshield, reservoir, high flow. I did keep the pvd VARIO vision block. It is on its 3rd psu with a Seasonic Prime 1600TX sitting in New box shrink wrap. The PL2s have gone into the older box still running happily - 5950x, 32GB DDR4, 20GB 7900XT under water with all AquaComputer stuff too, just 2x360/2s, a couple of Samsung Evo Plus 2TBs and it's still a strong box.
'Desmo' dropped 64GB ram as I could not get it stable, but 64GB of Ram and 12TB of Gen 4/5 Samsung solid state is nothing to be sniffed at either. I have migrated one of the 990 Pros to CachyOS and it runs pretty well.
Tomorrow I'll line up the upper rad with the side rad, then It'll be a case of stripping down draining and lining up the lower fat rad while waterblocking the Radeon.