Chris, I'm with Malcolm, doesn't look like coma as the drift is all the same way, suggesting tracking/guiding errors.
The thing with guiding is that you need your polar alignment to be spot on, guiding is not the solution to fix rough polar alignment.
How did you stack the images? To me, it looks like the stars are figure of eights, as if the stacking didn't register the stars correctly.
For focus, I use a Bahtinov mask...inexpensive and not ambiguous. Collimation being out will result it seemingly out of focus or off-round shaped stars over the length of a sub too.
It _is_ possible to get decent results with low-end DSLRs, many have been using 1100D, 450D, etc for years, but getting the results doesn't happen overnight, you've got to get a feel for your kit...I tinkered with guider settings for new Moon after new Moon and slowly got to grips with judging the seeing and setting the guiding accordingly.
But seeing that you have a full frame DSLR, go with that instead. Several reasons for me saying that...firstly, the sensor performance will be superior. Then you can control it with BYN. But also you get a much larger FOV to see what your scope is doing. It doesn't matter if you crop the corners out aesthetically, but the corners might give you some interesting information about how your scope and mount are performing. For example, I would expect to see coma and/or spacing artifacts in the corners of a full frame sensor, but you'd also be able to judge your tracking by seeing if they all go the same way (tracking or spacing issues) or are radiating from the centre (coma). Oh and the crop sensor doesn't give you more magnifcation, the focal length is really the same, the crop sensor just gives you less FOV to work with
Enabling dithering will help get rid of random and DSLR pattern noise, but it won't help if the basics aren't nailed...you need to balance your scope well for the region of the sky you will be imaging. Your polar alignment needs to be spot on. Focus precisely. Try taking shots unguided and see how long you can go until you get star trails. Whatever it is, aim for longer
I'd aim for 30-60s before enabling guiding. Guiding is only really good at correcting RA/Dec for deficiencies in the manufacturing (periodic error in the mechanisms).
Hope all this helps...you're doing well for 6 months in, just hang on in there and take one step at a time...