I’m level 50+ and have all but three or four certs, and an S level reputation. My goto for money and XP are Medivac missions. I don’t skip anything, climb to altitude, cancel IFR and program the destination airport into Direct-To. Set AP up at a good altitude and set heading as shown on the Garmin. Sure it’s not realistic, but I do those sorts of flights on my own time in free mode. Easy way to get lots of cash fast, and also level up.
Another thing, I avoid any mission with an aircraft I’m not super comfortable with. Helicopters? Nah, not yet!