Your focus is good so that's 99% there. It's quite a long exposure for an alt/az and because it's closer to the SCP you're bound to run into field rotation as well. It's hard to tell without seeing the whole field but it's probably a combination of things. Changing the camera won't resolve anything.
First thing would be to get a cheap star tracker or polarie so you can track the sky for 30s or 60s reasonably well.
From there on you'll be able to identify if you need to stop your lens further to mitigate coma or other.
That's where I'd start. But you're pretty close IMO. Keep at it.
