> I tried really hard to get it on 32GB but sadly the motherboard only has two slots and only supports 8GB DDR3 modules.

Buy a refurbished Xeon workstation with DDR4 RAM. If you're in the US or China they're basically free and if you're in the EU, they aren't too pricey (like you can definitely find ones for less than 200 EUR). Slap your double NVMe expansion card, your two NVMEs and your 3 3.5" spinning rust in it. If the workstation doesn't come with a Xeon with lots of cores, just buy one on eBay for $25.