I've watched Alex O'Connor's work on YouTube for awhile and came across this episode of his podcast. Y'all are welcome to react to the episode as a whole of you wish, but the Catholic bishop talks about the summum bonum at around 14 or 15 minute mark. His explanation is exactly how I understand the term: keep asking "why?" and the answer at the end is your summum bonum. I can see the answer being "it brings me pleasure" Her didn't, of course. But I thought this was a good explanation of the term summum bonum. Which is why I balk at Dewitt's "life is the greatest good" because it seems a tautology to answer why I do something as "because I'm alive."
Anyway, it's an interesting conversation.