The concept of extenal outputs is weak link of Vmix software. Can you check how it is working in Livestream Studio? In LS I have possibility to add any number of outputs (for Decklink Quad 2 - 8) and it is much more easy to route inputs to outputs. Please, check LS and add possibilty to have any number of outputs.

For example: for simple setup I need

3 outputs = 1 for central screen and 2 for left and right screens
1 output = program output
2 outputs for speakers monitors

Looking forward to hearing from you.

Regards,
Ilya