If your project is too big, usually it feels clunky, slow to respond to anything. Takes forever to load.
If you want to break it up, I would start by breaking up your sequence into smaller sequences. Just duplicate the sequence, a few times, and start cutting it up.
If you want to shrink the project, do the same thing. Open your project, do a save as, and then cut out alternating sections in the projects.
Typically I’ve found the media in the project less influential than the size of the sequences, which is why a lot of us work in “reels” for bigger projects.
As for your problem, it is just as possible your graphics card is dying, or an update snuck into your box, which is not happy with the older version of Resolve.
(I’m surprised you can run resolve 9 on a MacBook pro)
Glenn