Should a handyman business focus on small quick jobs or scale up to larger renovation projects?

Sounds similar to how my brother and I started out. First off, congrats. The problems you're facing are great struggles to have. 

I'd say, don't dream of going back to smaller jobs. But don't rush to double your size. Get really good at what you're doing now. Focus on efficiency and quality and building out documented processes for everything (including how to avoid issues you may have faced in the past, if any). If you don't get that dialed in now, it won't be any easier at twice the size.