Either route is beneficial and we always can switch to something else. At the end of a day it is not about the engine, so whatever gets the work done.
Yes, jams are quite helpful to understand the scope and how to finish something workable under a deadline. Pet projects can last decades otherwise, if not careful.
Being an expert is not crucial, as long as you can produce something. Unless your goal is to be the expert, not to make games (I mean goal to be hired for the expertise, compared to going indie).