re: adding new skills to differentiate jobs
It seems easier to add traits, so maybe add traits that can be gained by doing a non-sex job. For example, "Good singing voice" might be gained if a girl does the singer job a lot. That trait could add to the pay. Maybe continue to use the Service skill for a variety of jobs, but slow down its improvement and have job oriented traits have a bigger impact. These could have levels as in EX such as "Good voice 1", "Good voice 2", etc. Other jobs may be as straightforward as "Barmaid 1", Barmaid 2". Maybe have job specific items as well such as a "Manual of Mixology" which helps bartender jobs or "Scrub brush" for Cleaning.
re: having a text/config file for jobs
The more things a player can tweak in a config file, the better. Pay/expense for various jobs is already in the config file, but I don't think adjusting it does anything. Which type picture shows may be much harder to give players the ability to change, but what might be nifty is the ability to set defaults. For example, let's say there's a new category of picture called "Clean", but I don't want to go through all of my packs and add/edit pics. A config file that allows me to say what should be used if Clean isn't found (maybe some want Ecchi, maybe some want Profile).