> knowing full well that [Delilah] will betray [Samson]
It is not clear from the text that Samson is aware of the presence of Philistines the first three times. We are told they are lying in ambush in an inner chamber. We are not told that they make themselves known, attempt to subdue Samson, or interact with him in any way. Instead, we are told that Delilah announces their imminent presence, with no follow-through. (And why, after all, would they dare come in until Delilah has established that he is weak and harmless, given his reputation for strength and bloodthirst?)
I don't believe Samson understands Delilah is betraying him. Not until the end when he's told her the real secret. Until that point, he may only receive her behavior as, at worst, a taunt and, at best, an attempt to pursue a kind of relational intimacy in which he makes himself vulnerable to her. And indeed, that's exactly where the story goes. He eventually makes himself vulnerable to her, both emotionally and physically.
A shallow reading that concludes the characters must be dumb and naive to explain the ridiculousness of the plot is rarely the most appropriate reading. Frequently, the characters and nuances run much deeper, and a more careful and thoughtful reading is required. Ancient literature is stylistically terse compared to modern literature. Space on the manuscript is precious and not given over to things that the reader is meant to infer by reflection. And whenever a phrase or pattern is repeated, that means it is very, very significant. In this case, Samson's interaction with Delilah is not a one-off inexplicable behavior. It is one episode in a series of episodes, both preceding and following, where consistent patterns of behavior develop and escalate (e.g., his vengefulness, his vulnerability to women).