All you need is SSH and SSH config. There are a lot of tutorials out there, but I find many of them are based on older versions of SSH which lack a bit more sugar around proxying.

Essentially you will add a directive to SSH config for the NAT host, and the host that you want to access. In the directive for the host to access, you will specify that you're proxying through the NAT host.

You can then leave out all of the port forwarding options when connecting to the target host, SSH will pick that up from the config file.