How to set up RAID using 10Gb ethernet - without the internet.

I am still waiting for you to describe how you connect to the Internet generally.

All the IP addresses on your home Network are likely to be from one of three ranges of strictly local IP addresses. Those are no good for talking DIRECTLY on the Internet. Your Router 'acts as your agent' on the Internet, and translates those local Addresses to its Own IP address, essentially saying, "when you send the answer, send it to ME, the Router, and I will return it to my requestor ... and I won't tell you to where."

When you set up another local device (easiest to imagine may be a Network Printer) its packets are sent DIRECTLY across only your local Network through local connections, Switched through your Router. The Internet could stop working and you could still print just fine.

When you set up a Networked Attached Storage (NAS) device, its packets will also go through ONLY the Switch function of your Router (or other stand-alone Switches) and be sent DIRECTLY between your NAS device and your computer.

One more rule to remember: Packets going toward the Internet are sent through the TOPMOST, WORKING, Interface shown in:

System Preferences > Networks

You COULD choose to connect ONLY your NAS to 10G Ethernet directly, and NOT through any Switches/Routers. [Cross-over cable is not needed by modern equipment, it all adapts, but all eight wires IS required.] On your Mac, make Wi-Fi topmost. Then Internet traffic can be seen to go over Wi-Fi, local NAS traffic will go where the local IP address of the NAS is available [10G Ethernet] which is not the TOPMOST, but is a WORKING interface.

This setup is not optimized for best Internet performance, but can allow you to understand in a visceral way that your NAS traffic and Internet traffic are not being mixed, and the Internet is not being used for your local File Transfers.

[if you do this, you may need to manually configure your 10G Ethernet addresses if it does not immediately work, but that is a straightforward, one-time process that Readers can help you with.]

Maybe these with help

Direct-connect NAS via ethernet? - Apple Community

This one uses a synology NAS but I suspect something similar would work with Qnap NAS

what is your ordinary connection to the Internet, using 10G bit Ethernet, or using a different Ethernet port, or using Wi-Fi?

By far the biggest problem when using the Internet is caused by "the nut behind the wheel". approving software for installation when it is not what was expected. Individual stations on your home Network are at LESS risk that the one you use for web surfing and reading emails.

Your hone network is extremely unlikely to be attacked form the Internet directly, because your devices are not reachable from the Internet at large. All the IP addresses are private and un-routable. Everything on your home network is protected by the Network Address Translation firewall inside the Router you control. Your Router never allows anything from the internet to get through, unless it is a direct response to a request from one of your network devices.

The simple connection from one Mac to one NAS shows its drawbacks when you want to also connect your spouse's computer to the NAS.

Now you may need a10G Ethernet Switch, to make multiple connections and maintain order.

In many cases, the Router (especially one provided by an ISP) may not be able to do this at 10G speeds, so you may have to add an additional Switch just for 10G traffic.

ThunderBolt connection is simpler to get your head around, but slightly less flexible (can't easily support additional computers on your Network).

it appears the QNAP device can transfer just shy of about 1200 M Bytes/sec max transfer speed with either Thunderbolt-3 or 10G Ethernet, so no gross performance penalty.

The complex QNAP device is something you can "grow into" and take advantage of its more advanced features later. Like buying a high end camera for taking snapshots now, presuming you will want to do much more with it later.

Remember that having large 10G Ethernet or ThunderBolt PIPE (with the ability to transfer data at up to 1200 M Bytes/sec) does not change the fundamental speed at which data can be read off rotating magnetic drives. To read more data off a 7200 RPM drive, you still have to wait for the platter to spin back around.