How do established freelance illustrators build systems and raise rates?

[Oliver_T]

I've been building a freelance illustration career on the side for about a year, landing small client projects through word-of-mouth, but I'm completely overwhelmed by the business side and feel stuck at a low income level. I don't have a streamlined process for contracts, invoicing, or managing client revisions, and I chronically undercharge because I'm afraid of losing work. For established freelance illustrators, what systems or tools have you implemented to create a sustainable business structure? How do you determine your rates confidently, handle difficult client requests that fall outside the initial scope, and most importantly, find consistent higher-paying clients beyond your immediate network without burning out on constant self-promotion?