I am by no means an advanced user when it comes to this kind of stuff but just mentioning Tailscale as an option if it fits your use case. It effectively gives you site to site tunneling with the option of zero trust controls, and is extremely simple to configure to work in what I believe is your intended use case.

I’d give it a read through and see if it works for you.

+1 for Unraid, as it’s what I use, but truenas I believe has array options which support your plans no problem. It’ll do a similar software raid solution which allows you to modify the drive pool later on.