Yes, I think you’re right, single gene diseases will be the easiest to treat and that will happen first. But, once the method has been established as safe we may be able to treat multiple gene defects at once.
This article is on how DISC1 gene causes problems with a protein. It’s possible other genes cause problems with the same protien. That’s a possible target for gene therapy, or they may be able to develop a drug to increase levels of the problem protein.
This one is on the gene Neuregulin.
Then there’s the C4 gene, possibly correcting that could be helpful.
Someday, we will be able to have individual treatments that sequence your genome and correct just the genes that have errors in them, but that is a long way off.