I have four old 24gb Nvidia cards. They're not great but they're not useless either. The problem is that I haven't really figured out a good way to actually use them.

Genuine question; would anyone here recommend any specific motherboard to best utilize these cards?

Depends what you want to do and which cards you have, but usually going with any older (3rd gen+) threadripper pro setup will give you a lot of pcie lanes.

I myself run with gigabyte trx40 aorus xtreme, but since it's regular threadripper (not pro) with 4 GPUs 2 of them will run at x16 and two of them at x8 speeds