It is possible to do what you want.
My main job is as a survey pilot, doing contracts in Africa and Latin America on turboprops. But in my time off I live next to a busy European GA airfield with large numbers of privately owned single-pilot jets (Citations, Phenoms). I get freelance work on them as second-pilot. It helps that I previously flew as a biz jet pilot so am familiar with the scene. My TRs are now all lapsed, but I don't need a current TR to fly as second pilot on a single pilot aircraft. The money I earn is just a top up to my salary as a survey pilot, and it is good fun to fly jets occasionally.
I do also know of a female pilot who has her own successful IT consultancy. She pays out of her own (deep) pockets for 3-4 type ratings, and freelances maybe 6-8 weeks per year. I doubt that what she earns pays for what she has invested in training, but for her it is more of a hobby I think.