Don't need display port, GPU, WiFi, Bluetooth, GPIO pins, etc - just a CPU + lots of RAM + an ethernet port to get stuff in/out.
Alternatively, is there a practical way to attach external DDRAM to an existing 64-bit SBC, perhaps using the GPIO pins?
I have zero electronics knowledge so forgive my questions if they are stupid.
As for attaching memory, GPIO probably gives you a transfer rate on the order of a few MB/s. There are various things like SPI-connected SRAM chips, but those don't provide a lot of memory (think KB, not GB).