How much do you want to spend?

If you go for a Raspberry Pi have a look at Terrapi cases as well the obvious Argon ones.

Another option would be a Zimbaboard. It is more expensive but it has dual SATA connector (you need to buy a Y cable with the Zimbaboard) and there are 3D print designs to create a single unit, e.g. https://www.printables.com/model/224057-zimaboard-dual-hdd-stand.

I’m not sure about PoE and a NAS. Will a PoE HAT or similar provide enough power for the board and the drives?

OpenWRT support on GL.inet devices seems to be complex. The following is my understanding of the situation.

GL.inet have an OpenWRT fork on GitHub https://github.com/gl-inet/openwrt This is what is installed on GL.inet devices.

The OpenWRT developers in due course try to work out how to port mainline OpenWRT onto OpenWRT onto GL.inet devices.