Images that convey information should have alternative text that conveys the same information. Unfortunately that is not practical for a complex diagram. A workaround is to link the image to a long description in the form of an accessible webpage or downloadable document. The image alt text should then describe the link.
I do think yours is the correct answer, but I thought of another possibility. What if the content experts said that all the information and relationships expressed in the Venn diagram are duplicated in the text? And the diagrams are made artifacts. I would have to rely on the veracity of the content experts, but I also have to rely on them to create an accurate long description document as well. Just a thought and thanks for your response.
A thumb rule to remember - "accessible" conveys the content author's / authors' intent. Consequently either of your approaches would serve. But, fundamentally, what "a_C.." shares would let you do either.