FS6706T

The one that you list may have some heat issue, note that none of these pre-built nas will give you the x4 lane, pretty much x1 (with 1 slot may be x2). Beelink Me also an option but it may have issue to power all 6 nvme with RAM at once. ASUSTOR FLASHSTOR 6 is another option but it is quite pricey.

I don’t think the Flashstor supports expansion units. It’s kind of designed to be an all-NVMe, all in one box, so there’s no port for the ASUSTOR expansion drives. If you need something you can expand later, their Lockerstor models support the add-on units, but the Flashstor is more of a “max it out