What we do as technical communicators is not only “technical writing” but also a lot of “information design and development” as well.
So here is a certificate program by a good school, University of Massachusetts Boston: Information Design and Development Certificate.
The program’s web site shares this: “This program offers the fundamentals required for technical communication in the high-tech industry, as well as more advanced topics for developing and delivering effective technical information. This non-credit, online certificate program consists of four required courses, two electives, and an optional internship. Upon successful completion of each course, you will receive 3.5 CEUs.”