Given that you've said 'workstation', if you've got a spare $5000, a Threadripper Pro comes with 128 PCIe 5.0 lanes.

This means you can get a motherboard like the "Asus Pro WS WRX90E-SAGE SE" which dedicates 104 lanes to seven PCIe slots and 16 lanes to four M.2 slots.

For more like $3000 you can get a non-Pro Threadripper; the "Asus Pro WS TRX50-SAGE" has a more restrained 48 PCIe 5.0 and 32 PCIe 4.0 lanes, meaning the board's five PCIe slots and three M.2 slots have a mixture of speeds and lanes.

The rest of the market seems to think you just want to plug in one huge four-slot GPU and perhaps one other card.

Now we're talking - thanks!

(ps. I don't suppose they make a "supersized" version of that board with a gap beside the first one or two GPU slots? So you can install a couple double-width cards without losing the underlying slots? Or a good source for a single-width, high-end GPU like the Inno3D RTX 5090 iChill Frostbite Pro?)