The way I see it, there's a level of complexity above which SSH becomes too painful to administer. So now what? Well, unless you find something inbetween you are up for setting up a full VPN and managing the complexity and security of that. And if your network needs are quite fluid, that itself is going to be a pain. Zerotier to me looks like that "something inbetween". But I'd be interested in Sam's comments.