Markan priority is just about the most universally agreed-on fact in New Testament textual criticism iirc.
One simple example is Mark 2:26, where Mark names Abithiar as high priest when David entered the sanctuary, when 1 Samuel says it was Ahimilech. Both Matthew 12:4 and Luke 6:4 avoid the error by omitting the name of the high priest. In terms of priority, which is more likely: that Matthew and Luke were copying Mark and spotted and removed the error (as later copyists of Mark itself tried to), or that Mark was copying Matthew and decided to add in the the wrong high priest?
Also, supporting Markan priority has nothing to do with whether Q existed or not. Lots of scholars are sympathetic to the
https://en.m.wikipedia.org/wiki/Farrer_hypothesis which argues that Luke simply used Matthew.
