Maybe you can make new vÃdeos of the segments you are trying to multicam, it might be that even if you selected only the segments FC is strugling because it’s really heavy files.
I don’t know if that is the problem but it might work.
Multicam is a very intensive proces, I recently made a 8 multiclip in my older tower (2006 2.66 x 2) and I didn’t play. It worked on a newer iMac.