Rob, what exactly model of the router do you use?
Connecting the HDD directly to the router is not the most correct way, because routers have very weak processors, and data transmission via USB protocol uses certain processor resources. Generally speaking, the correct practice is not to load the router with any additional tasks except important gateway and wireless ones: connection with the provider, firewall, gateway to the local network, DHCP server, Wi-Fi access point. All tasks beyond these are simply overloading the weaker hardware of the router, which is why it heats up more and works more unstable.
Even as a LAN switch, it is better not to use a router, but to buy a high-speed switch.
In the context of IT, you need to learn how to cook properly.
You're not putting fish in compote, are you?
I do not need to transfer large files via Wi-Fi. I do it through the wire. It is faster and more reliable. But in the case of wired connections you need to choose the most effective methods available to you. I have already shown here how and at what speed I import video files of several gigabytes and even tens of gigabytes into the iPad. For such purposes, I use Samsung SSD
connected to the iPad Pro via an external powered hub.
And Synology NAS I use to store archives and libraries of media materials.
But the NAS is connected to a local switch in two gigabit ports. When organizing a RAID-10 array, this provides a wired network write speed from MacBook Pro to Synology of about 120 megabytes/sec. The reading speed from NAS is naturally higher.
To speed up the processing of imported video on the iPad, I turn off
iCloud Photos before connecting the SSD and turn it on after
the file is imported into LumaFusion, that is, before exporting the result.