IMO you’re approaching this very wrongly. You’re not looking for an mITX board. You want the performance and expansion of a thread-ripper board, for cheap, and in a small form factor–and it doesn’t exist. No matter what you end up doing you will have to compromise in the mITX form factor.

Additionally, mITX x16 slots aren’t always 16 lanes of PCIe. To save on space, or to compete with other manufactures, many of them are PCIe 3.0/4.0 x16 at 8x speed. Many boards that are full speed are expensive. They start at around $200. Which runs you into another problem. The number of PCIe lanes is limited by both the board and the CPU. mITX boards are designed for low power systems. They’re don’t have an infinite number of PCIe lanes. Usually about 16 for mITX boards… So if you have an 2x M.2 at 4x speeds, your x16 slot is limited to 8 PCIe lanes even if it has a full 16 lanes available to it, but the way that you’re using it you’re paying a premium for hardware you can’t fully utilize. Which means even if you do use a PCIe x16 to SATA bifurcating adapter, the most you can use is 8 additional drives at 1x speed each.

Not to mention the power limitations of your mITX motherboard. You need a minimum of 25W per HDD for peak power consumption, otherwise your rig will be unstable during heavy load. That means you need a minimum of 200W just for your hard drives, not including your motherboard, CPU, onboard graphics, etc. So likely between a 350-400w mITX power supply which can be a little hard to find for mITX boards sometimes.

Frankly speaking there are a significant number of issues with what you want to build and how you’re going about doing it. There’s no advantage of doing an mITX build for what you need outside of size. Everything else about mITX for you is a limitation.