The SSH agent symlink tip will not fully work. If you connect with a second SSH session and then disconnect it will point to a stale SSH_AUTH_SOCK that doesn't work. The only way I found to solve this is by having a prompt command that refreshes it every time your prompt is rendered. So worst case is you have a broken ssh-agent for a single command.

Add this function to your zshrc/bashrc:

  # This function refreshes some env vars that go stale in old tmux sessions
  # It must be run as a preexec function in zsh or a PROMPT_COMMAND in bash
  function refresh_env {
      local ssh_auth_sock=""
      if [[ -v "TMUX" ]]; then
          ssh_auth_sock=$(tmux show-environment | grep "^SSH_AUTH_SOCK")
      fi
      if [[ -n "$ssh_auth_sock" ]]; then
          #shellcheck disable=SC2163
          export "$ssh_auth_sock"
      fi 
  }

Then for bash, add this:

  if ! [[ "$PROMPT_COMMAND" =~ refresh_env ]]; then
    PROMPT_COMMAND="refresh_env; $PROMPT_COMMAND";
  fi

Or for zsh, add this:

  autoload -U add-zsh-hook
  add-zsh-hook preexec refresh_env