Author of the blog post here. The standard shortcuts are ctrl+` for toggling the panel and ctrl+shift+` for creating a new terminal. This is what I use on top of that:

{ "key": "ctrl+`", "command": "workbench.action.terminal.focus", "when": "!terminalFocus" }

This will focus the terminal if the focus is anywhere else, if the terminal is focused it will close it.

You can add this one if you want ctrl+` to only toggle focus between the terminal and the last editor:

{ "key": "ctrl+`", "command": "workbench.action.focusActiveEditorGroup", "when": "terminalFocus" }

On tmux, Joao from the team did a write up on configuring persistent sessions in the terminal using tmux https://medium.com/@joaomoreno/persistent-terminal-sessions-...