This is technically not true, the FCC can and does enforce spectrum usage rules. Whether they will expend resources chasing down your router or your unlicensed GMRS is another matter.

You can run pfsense on a gig of memory if you arent using Snort or pfblocker. I had this on a vps for an email gateway, worked fine. Adding block lists or services gooses memory usage though

Can you build it yourself like you can for Lineage?

There are also the BSDs