As I mentioned earlier, it is possible to do such things within the .tmux.conf, but not practical. Here's one of my real tmuxp configs:

    # How to use this file:
    #   tmuxp load /path/to/this/file/main.yaml
    # - Specifying the .yaml extension is optional
    # - Specifying the path is optional if file is in ~/.tmuxp
    #
    #   tmuxp --help for full help.
    session_name: main
    suppress_history: false
    start_directory: '~'
    shell_command_before:
      - unsetenv SHELL_CONFIG_LOADED
      - source ~/.alias
    windows:
      - window_name: '*scratch*'
        layout: tiled
        focus: true
        panes:
          - cd ~/downloads/git           # Top-left
          - cd ~/downloads/git/emacs     # Top-right
          - focus: true                  # Bottom-left
            shell_command:
              - bob
              - sleep 2
              - tclr                     # Clear tmux status indicators
          - shell_command:               # Bottom-right
              - cd ~/.emacs.d/elisp/org-mode
              - module load gcc/6.1.0    # Needed for ob-fortran tests to pass when doing make test for Org
      - window_name: foo-1
        layout: even-horizontal
        shell_command_before:
          - bob
          - cduh
        panes:
          - null
          - null
          - null
      - window_name: foo-2
        layout: tiled
        shell_command_before:
          - bob
          - cduh
        panes:
          - null
          - null
          - null
          - null
      - window_name: uvm_lib
        layout: tiled
        panes:
          - cd ~/gitlab/bar_uvm_lib
      - window_name: sandbox
        layout: even-horizontal
        panes:
          - null
          - null
          - null
      - window_name: hugo
        layout: tiled
        panes:
          - shell_command:
              - cd '~/downloads/git'
          - shell_command:
              - cd '~/e/elisp/ox-hugo/example-site'
              - ./srv
          - shell_command:
              - cd '~/hugo'
          - focus: true
            shell_command:
              - cd '~/e/elisp/ox-hugo'
      - window_name: nim
        layout: even-horizontal
        panes:
          - cd ~/sandbox/nim
          - cd ~/sandbox/nim
          - focus: true
            shell_command:
              - cd ~/sandbox/nim/chat
      - window_name: tok
        layout: even-horizontal
        shell_command_before:
          - tok
          - cduh
        panes:
          - null
          - null
          - null
      - window_name: zoo
        layout: even-horizontal
        shell_command_before:
          - bob
          - cduh
        panes:
          - null
          - null
          - null

I don't do it within the .tmux.conf - I use separate scripts, one per project.

I understand. But the point I am trying to make applies there too. A script to do what I am doing in the tmuxp config I last pasted would be too unmanageable, and not as readable and reusable.