You may not be able to do RAID or other redundant/performant arrays with USB. You can definitely achieve a big JBOD array but it will be less resilient and slower than a RAID array. Enclosures often don’t cool as well so heat may degrade your disks faster as well. I did this for a while with some old disks and some $30 HDD toasters. I only put data on there I could afford to lose. I wish there was a standalone hardware RAID solution… like a NAS without the network. That would have a huge draw for hobbyists that don’t want to buy an expensive NAS. I’ve searched for this but haven’t found anything. Message me if you know of such a product! Maybe consider building your own NAS with an old PC. Way cheaper than a prebuilt and fun to build! I had an old Dell Optiplex 990 that is now a 32 TB NAS. Had to get a new case but it’s a decent backup to my Synology.
This right here. Since you can’t really configure the ISP router (1), DMZ is the way to go. The DMZ feature on home routers usually allows you to select a specific device or devices to add to the DMZ so make sure to reserve/set a static IP on your router (2) and configure the DMZ on your ISP router (1) to include only that specific IP. DMZ essentially forwards all ports to that device.