Some years ago I realized that if I had oodles of money to spend I would totally get someone to make a PCIe card with like several hundreds microSD cards on it.

You can buy vertical microSD connectors, so you can stack quite a lot of them on a PCIe card. Then a beefy FPGA to present it as a NVMe device to the host.

Goal total capacity, as you can put 1TB cards in there. And for teh lulz of course.