I'm not so sure about the no upside bit... he is trying to sell a book that is yet to be released... he is waiting for just the right time... a time like when he gets tthe country talking about what he might say, and then never get called to say it. That's worth Millions in book sales.
He doesn't want to testify, he wants people to want to read his book.
My personal read on him is that he views himself a patriot, and has always believed that his desire for intervention in the Middle East would be vindicated in the history books. His joining the Trump Admin was a gamble on being able to push his agenda, and he left when he sensed a risk to that image (viz. Drug deal). Bolton is the archetypal example of someone whose primary motivation is ideology.
you know what "true patriots" have very little of? Money. His time in politics is coming to an end and a new revenue stream for his retirement is needed.