A few more results before I call it a night :
I turned the filter settings up three notches to 4.3 / 5.8 / 7.8 / 11.7 dB and I finally got 60 FPS on lock throughout the Emerald Vale outpost of Outer Worlds. However I felt that those gains were a little high for an equalization stage, and I did see some weird behavior at PCIe 1.0 and 2.0 on the Windows desktop. That may have been signs of saturation.
so I dialed back one step and instead rose the DC gain by 1.5 dB (from -0.5 to +1).
Lo and behold, I got my best result so far. In fact I turned off V-sync in Outer Worlds and got 90 to 100 FPS, no dip lower than 80 FPS. As I was saying, just cranking the gains all the way up isn’t a viable approach.
I still haven’t messed with RX gains.
@Minnie : I think we know how we’re going to make your rig work (or at least work better). It’s clear that you’ll need to experiment with redriver settings on a slot-by-slot basis. There’s also the fact that not all your 3090’s are the same.
If I were you, I’d characterize my 3090’s first : get one working on slot 5, 6 or 7, then try them all and see how they behave. Chances are, 3090’s from different vendors (with different PCB’s) will perform differently on the PCIe bus.
Since you have 6 GPU’s, only 3 can go into “redriven” slots. Your GPU’s with the best signal integrity should go on slots 1, 2, 3 : no redrivers but closest to the CPU socket. The worst three GPU’s should go on 5, 6, 7 where you can amplify the signals.
I will likely do some endurance testing this week-end. Any excuse to play video games for a change 
(seriously though, I just don’t see myself starting serious work on a machine that I’m not yet confident is stable). I’ll keep posting if I come across anything more, good or bad.