There's not much else (other than Epyc) in the way of affordably priced motherboards that have enough cumulative RAM. You can buy a used Dell dual socket older xeon CPU server with 512GB of RAM for test/development purposes for not very much money.
Under $1500 (before adding video cards or your own SSD), easily, with what I just found in a few minutes of searching. I'm also seeing things with 1024GB of RAM for under $2000.
You also want to have the capability for more than one full speed at minimum PCI-Express x16 3.0 card, which means you need enough PCI-E lanes, which you aren't going to find on a single socket Intel workstation motherboard.
Here's a couple of somewhat randomly chosen examples with 512GB of RAM and affordably priced. they'll be power hungry, and noisy. Same general idea from other x86-64 hardware such as from hp, supermicro, etc. These are fairly common in quantity so I'm using them as a baseline for specification vs price. Configurations will be something with 16 x 32GB DDR4 DIMMs.
https://www.ebay.com/itm/186991103256?_skw=dell+poweredge+t6...
https://www.ebay.com/itm/235978320621?_skw=dell+poweredge+r7...
https://www.ebay.com/itm/115819389940?_skw=dell+poweredge+r7...
PowerEdge R series is significantly cheaper if you already have an ear protection
yes, an R730 or R740 for instance. There's lots of used R630 and R640 with 512GB of RAM as well, but a 1U server is not the best thing to try putting gaming GPU type pci-express video cards into.