As a consultant (as opposed to a pair-of-hands freelancer) you want to be providing solutions to businesses that solve their problems, at a higher level than "work on this software solution the client designed and just needs me to implement".
From that perspective, I'd argue that adding more technical skills on top of your existing ones will have diminishing returns past a reasonable level of competency.
Instead, focus on being able to provide business value. Learn how to take general problems a business has and find solutions for them. Not just code solutions, but new processes and procedures.
If you can solve an expensive problem for a business, you can command a far higher price than if you're just a pair of hands writing code. Those are a dime a dozen on Upwork, and clients have trouble seeing any difference in skillset.
From a technical skills perspective, machine learning and "big data" processing are in-demand skills with large enterprises, where you can make the most money solving problems.
I'd be glad to continue this discussion if you have questions about my answers. My email is in my profile.