Hi Lewis, I am not getting why it is a problem to have the camera under the tube..? This is the usual positioning for cameras on a Newtonian and generally means you don't really need any off axis balancing.
How are you focusing? By hand or electronic? On a star in software or using a Bahtinov mask and your eye through a viewfinder?
That's a great scope with much more aperture than any APO (you could afford) I am sure, so don't give up yet
Mike