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With limited vision myself at my place I find alignmaster not great. It is very good when I can see the stars, but my obstruction to the south hinders me. I would bet that the reason you have out of round stars is as stated earlier that you have differential flex.
You are imaging at 1800mm on a mount with high periodic error, I doubt you will get round stars with perfect polar alignment (guiding turned off). If you then turn guiding on and cannot get round stars it is due to differential flexture, which would be prominent because of your image scale. If I do a quick drift align and take a minute unguided exposure my stars will not be perfectly round (800mm focal length). But as soon as I start guiding the stars are perfect, and I have exposed to 20 minutes. With a relatively large steel tubed scope and a focal length of 1800mm I would have thought an OAG would work better, it certainly is better for me. Of course I have no experience at long focal length. (I imaging finding a guide star will be a pain)
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