First, I'd love to help you as you grow and expand your freelance career. It is truly my passion to help others do what they love.
As for your question, I like "WordPress Virtual Assistant". As a former agency owner, this speaks to exactly what you do. If you have a web presence (I expect you do), then you can go in to more detail about your freelance capacity, your skills, and how you can help agencies and firms outsource some of those administrative tasks that bog us down and cost the client hundreds an hour for a developer to handle.
Keep in touch,