I’ve used IPSec to connect to my home (and office when I had my own company) networks. It has never failed me and the client is built right into my iphone and mac.

FreeNAS can do it (either via AFS or SMB). Or you can just set up a Samba server with the right magic incantations and use that network share to do Time Machine. I use FreeNAS since I already have it on my network. I gave each Mac its own ZFS volume so I can limit the size independently.