Recommendation #1 -- Don't put your file server on your firewall.
As much as router manufacturers like to advertise this as a "feature", think about it, what box is most exposed to hackers and likely has the oldest kernel and application software running, and potentially weakest software from a security perspective due to the hardware's limited resources? Bingo.
Recommendation #2 -- Get a separate xDSL modem
"Obviously" isn't obvious, as there are many ways to get transport. Even among xDSL technologies, there isn't just one way to get the signal delivered and one piece of hardware may not be compatible with another. A separate modem gives you flexibility in hardware choices, as well as being able to replace the modem independently from the router.
Now on to your "requirements"
USB 3.1 / C -- Welcome to the world of amd64/x86_64 motherboards with recent, upper-tier chipsets of PCI cards. I'm not aware of a router that supports either of those, and even if there was, probably not on the way you're expecting them.
4-bay HDD JBOD? -- You go to all the expense of multiple drives but aren't using some type of data redundancy? Hmmm, not the wisest path. Look into RAID or ZFS.
Mount on Linux -- Have you considered that you may need extended attributes and the like? NFSv4 is something you should look into. You'll probably need kerberos as well. I don't think either are supported on OpenWrt.
SFTP -- Might want to look into the differences between scp, sftp, and rsync. Not knowing what you're trying to achieve something like NextCloud might be a simpler approach that custom scripting rsync.
Recommendation #3 -- From the above, run your file server on amd64/x86_64 hardware on a ZFS file system.
you could consider running tgt (iscsi) if you need the drives to behave like a local disk, but for tgt you need to enable kernel async io (aio) from config menu.