Three suggestions not mentioned yet:
On my office Intel Mac (mini) the screen resolution has a huge impact on how good the audio performance is. Seems that the scaling puts a lot of load on that old machine. My office i7 is completely unusable for VSTs even with large buffer sizes when connected to a large monitor.
Please try to use the standard resolution, if you don’t already.
Please also test with a stripped-down gig file to exclude a RAM issue.
Have you made sure the audio buffer settings haven’t changed?