I doubt you’d necessarily get more readers if you had a doctoral degree in theology. There are lots of popular theological books that are written by people with few to no educational credentials; usually, what matters most is the popularity of the author prior to writing the book and/or the popularity of the message they’re trying to offer. (The exception, of course, is if you want to do theological research or write books for scholars of theology. In that case, you do need a credential).
Furthermore, why pursue a D.Th? The M.Div would seem to be a more fitting degree for you.
Personally, I don’t think a desire to write books in the area is a good reason to get a doctoral degree (unless, of course, by writing books you mean you want to do research and write academic/scholarly books, as I mentioned).
As for whether you’d struggle: yes, because everybody struggles in a doctoral program, regardless of their intelligence. Intelligence, honestly, is not the marker of someone’s success in a doctoral program. Most people who get into doctoral programs are smart and were at or near the tops of their college classes. Hard work and persistence, as someone else mentioned, is what separates successful students from those who are not.
I have a terrible short-term memory and I have a PhD. Of course, like everyone else, I struggled in my PhD program (not with the content or material, but with the challenge of doing so much at once).
Value vs. effort is completely a personal determination. It really depends on how much having a doctoral degree is worth to you with respect to your career goals and personal motivations for getting one.