Gravitational Teleport is frequently used for SSH behind the firewall as well, but also tackles many other aspects of SSH for organizations.

It's Open source! https://github.com/gravitational/teleport https://gravitational.com/teleport/

Disclaimer: I am employed by gravitational

if you want a dedicated peer to peer vpn, zerotier.com is great.

Plug: If you want adhoc peer to peer port forwarding, you can try https://github.com/selvakn/p2p-port-forward (which uses zerotier-sdk)

sshreach.me is somewhat similar but solves different kind of problems (I'm the author so this is a shameless plug)

https://sshreach.me/