Irene,

There are pros and cons to being a contractor with experience in many domains and versus one who is very specialized.  You seem to be most disappointed with a lack of increasing bill rate. You're right that some specialists can demand a rather high rate, but remember that comes with a trade-off.  If that person ever loses their job the number of open positions that will work for them is much much smaller.  As a contractor who has experience in many domains and many project types, you should be extremely versatile.  In fact, I'd argue that your leverageable BA skills are typically more important than detailed domain knowledge since a BA with a large breadth of experience has more tools to learn a new domain more rapidly than a BA that has less cross-functional experience.  Still, this is a decision you will have to make.  Personally, I'd take the breadth of experience.

Since you specifically mentioned SAP consulting, I will share some very specific thoughts.  I have a close friend who has spent the last 20 years as an SAP consultant in the Business Intelligence space.  He has done extremely well.  He started his own SAP consultant placement firm where he staffed SAP projects with SAP consultants.  Eventually he sold the firm to a company that brought his team in-house.  They brought him as well to continue to expand the practice for them.  So far so good, right?

While talking over a couple of beers just last week he told me that he now feels he's in a dying field/technology.  SAP is not the leader it once was with so many other BI firms (including Tableau) adapting, changing, and delivering powerful solutions.  It's a much more crowded space and he fears his highly specialized experience will soon be a detriment if he needs to find new work.  Let this be food for thought as you weigh whether you want to be very specialized or very broad in your experience.