If I am using a hyper-expensive 2019 Mac Pro for actual making money kinda work my MPX slots might already have GPUs in them; I doubt I would want to just pull those GPUs and shelve them, more likely I would buy an all new 2022 Mac Pro (last of the Intel models) with a pair of those new GPUs, and dedicate the 2019 Mac Pro to on-demand rendering...
Zero reason to place PCIe4 or PCIe5 SSDs in a Mac Pro that only has PCIe3 slots...
If I were using a Mac Pro to make a living, I would most likely have bought one or two new machines thru that eight year period; and I would have the "old" machines dropped into a renderfarm...
Regarding AMD / NVidia GPUs, if one cannot read the writing on the wall, which is in huge bold neon lettering, "NO THIRD PARTY GPUS", then I don't know what to tell you...
Regarding Apple add-in GPUs, only time will tell, but nothing we have seen so far indicates that will be a thing, the power behind the Apple SoC is the immediate onboard interconnectivity with the rest of the SoC...?
If the current Afterburner card (and I do not see Apple releasing a newer version) stays at US$2k, then it might be better to just spend that two grand on a M1 Max-powered Mac mini...?
"Afterburner is a PCI-E based accelerator card that handles the decoding of ProRes and ProRes RAW video codecs in Final Cut Pro X, QuickTime Player X, and other supported third-party applications.
(The M1 Max SoC) Media Engine designed to deal with video encoding and decoding in hardware. It can process H.264, HEVC, ProRes, and ProRes RAW content, with the M1 Max specifically having a video decode engine, two video encode engines, and two ProRes encode and decode engines."
Looking at your sig, it looks like you have CPU options available, right up until you top out with the 28-core Xeon; after that you would be hoping for Apple to do a final Intel refresh of the 2019 Mac Pro, which would require an all new motherboard...
If Apple decides to fully refresh (internals-wise) the 2019 Mac Pro as the last Intel product in the entire Mac lineup, I would expect an announcement that it would be the final Intel model (get them while you can)...
The trick is for Apple to figure out what would be the minimal PCIe slot setup they could get away with, yet still meet the needs of those who actually utilize the PCIe slots (looking at the audio crowd mainly?)...