This issue has been quite prevalent since the August 2019 patch. We have a thread running on it over at Answers HQ here. When the user tries to access the gallery and/or their Library the game will freeze but more often than not, crash.
SimsQARobo has helpfully suggested accessing the Gallery through a web browser but there does seem to be another fix which is working, it involves trashing a game file, EAWebKit.so, which rebuilds itself after a game repair.
Right click on the Sims 4 in your Applications folder and select Show Package Contents
Navigate to Contents > Frameworks
Drag the EAWebKit.so file to the Trash
Open Origin, right click on the game in your Games Library and select Repair Game
The file will be recreated after the repair and you should now be able to access the Gallery and your Library again.
As well as the Trackpad issue appearing since the 18 June patch we have a new lag issue in CAS that is pretty awful and affecting Mac users with Intel chips, judging by the posted specs. of some. You can see a video here of the issue.
Thankfully there are some really helpful Mac Simmers in the thread, candiecoatediv and twin-reverb-sims, who are being really responsive to requests for more information and actively trying suggestions. Twin-reverb-sims has suggested that doing the following has helped to minimise the lag a little:
Go into System Preferences > Accessibility > Display and check the box next to Reduce motion.
Go into System Preferences > Display and change the resolution towards larger text.
Please reply in the AHQ thread if you are experiencing this issue in CAS since the patch with your Mac information. And if you have any further suggestions which may help others, then definitely please post.