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?
This seems to have worked for the older devices, but I don’t know about the newer devices, for example far as I can tell the “Flint” doesn’t have mainline support despite being over a year old.
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.
I suggest that you read “In defence of swap” that various people have linked to. It includes information about swap size.
Here are swap size recommendations for from Red Hat and Canonical. You may not run oof these distributions but the information will probably still apply.
https://help.ubuntu.com/community/SwapFaq
Assuming you have 16GB of RAM 32GB is the maximum swap size you need if want to use hibernation. You can you less if don’t plan to use hibernation.
Have you ensured that your setup will pass email authentication processes?
It has been a long time since email from random hosts is accepted for forwarding or delivery. This Wikipedia may help https://en.wikipedia.org/wiki/Email_authentication