With Merlin (another open-wrt variant specifically for Asus routers) you can simply connect a hard drive via USB and then enable it as either FTP or SMB storage (even simultaneously).
In windows you can mount the ftp volume as a local drive, from mobile I use X-plore file manager (also via ftp), and on my Nvidia Shield I can mount it via smb and watch movies directly. 4k 10bit, no problem.
I'm using the Asus RT-AX88U with whatever Merlin firmware is current, but it's been working ever since, and straight out of the box. Other than activating the function and setting passwords, I really didn't do anything.