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Hi Josh,
I really don't think it has anything to do with the focuser or image train behind the focuser. A tilted focal plane can only cause defocus by itself. If other aberrations such astig are present they can be more easily observed under defocus, however a tilted image plane will have an axis in focus somewhere and in front of and behind focus elsewhere. You will not see the same aberration pattern across the entire image.
If the scope can deliver good images near zenith then the optics are probably ok, that is good news. As you slew lower in altitude things go pear shape literally, star dispersion increases making then look like little rainbows (try it with a OSC camera), the dispersion axis vertically aligned, this is really obvious at altitude below 30ish degrees. What becomes difficult to judge is the relative contribution of dispersion compared with OTA misalignment, and how much you're willing to put up with. High resolution imaging can't be done below 30 deg altitude, so maybe that's a bad place to be testing things. But the image should hold up well between 30 and 45deg
Best,
EB
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