An hour or two of ancillary work? Do it as a favor, not a billable project.
Negotiate scope, not rate. Try to make rate immutable.
You are charging too little.
Your rate is probably not a straightforward function of what your fulltime equivalent makes.
Get an accountant now.
Have a well-defined master contract and a separate statement of work (SOW) for each project.
Get acceptance criteria into the SOW, but don't make it ultra-specific or you'll invite arguments.
Incorporate. It's cheap insurance.
Get a lawyer and have your contracts reviewed.
Expect to work on your customers' paper, not yours. See: get a lawyer.
Reach out periodically to all your not-unsatisfied clients. You'll be surprised how much work this drums up.
Specialize on domain, not on technology stacks.