The issue not the available bandwidth from a M.2 slot, even if it seems to be a PCIe x1 slot, it is the controller:
You might get away with a decent SATA controller without a port multiplier—but good luck finding proper documentation for cheap hardware of this kind.
The safer hardware solution is to strap a SAS HBA to a M.2 to PCIe slot adapter.
But the honest answer is that you’ve outgrown what a motherboard “from a 6 letter Chinese brand” can do.
Mirrord Boot Overkill Monster strikes again…
Use a small NVMe drive as boot device (single!), and you have 6 SATA ports for HDDs. Beyond that you really need a better motherboard.