I totally agree. I have a very similar setup as OP. I have 4x AMD Instinct Mi50. They are 32GB each with 1TB/s memory bandwidth. I got very lucky and bought them at 220 euro a piece back when prices were sane. Now they cost almost tripple of that. For the server I bought a used X99 xeon that has enough PCIE lanes for all four GPUs, with ASUS X99 WS motherboard. The CPU was only ~22 usd from Aliexpress. The build made some small sense when prices were good. I just did it for fun. Now it makes no sense at all.
I did a very similar setup with 4x AMD Instinct Mi50. For the air intake I used a single Noctua Industrial PPC 14cm fan, with a 3d printed duct. In the end the cards don't produce that much heat. A single 14cm fan at low speeds was enough to keep them cool at 62 degrees max. A large fan is also more quiet. Also, removing the graphene pad betwee the die and the heatsink, and replacing it with a good paste, helped bring down the temperature by few degrees.
