Odyssey x86 M.2 B sata converter?

I’m using an Odyssey X86J4105800 as a home router running openwrt. I’ve already included an NVME drive in the M.2 M slot (to run openwrt and nextcloud) and was hoping to add additional RAID 1 storage capability (perhaps using Samba as a NAS or NAS-like option). I already have two identical 4 TB 3.5" hdd drives that would work, but obviously I only have 1 SATA port.

As far as I can tell the seeed M.2 to sata converter is only for the M key I’m already using for the NVME drive.
I’m wondering if it is possible to get a converter for the M.2 B sata slot to allow me to connect one drive to the built in SATA port and one to the M.2 B port.

Is this a bad idea? Should I just bite the bullet and get a separate NAS enclosure? I also considered a dual bay DAS enclosure, but I hear reports of people having issues these being less reliable when converting SATA to USB.

Any one try something like this or have any advice or ideas?