Guess it's time to let sleeping dogs/old men lie (or something like that).
Another option might be to send the Mini back to OWC and have them put a 1tb SSD into it and install Sierra, if you don't feel you can do it yourself. Of course, it's not free but it would give you a much faster system with room enough to put everying on the SSD (assuming all your files fit into 1tb).
It almost sounds like that is what you were expecting them to do originally, seems to me you posted that the new Mini had a 1tb SSD in your other thread.