“And I definitely don’t see why her minhagim should be the trendsetter.”
Why not if he doesn’t have a mesorah? As you wrote, if he has no mesorah, he can choose his minhagim, so wouldn’t the next best (to following his own mesorah) be following his wife’s (who does have a mesorah)? I don’t think that he has to if he doesn’t want to, but wouldn’t that be the logical choice?
You’re right that the most ideal choice for him to choose would probably be his
wifes Minhagim. But it isn’t that her minhagim bind him in a case where he has no mesorah; he may choose his own “derech” and she would still be bound to follow him. It may be convenient, considerate and accommodating of him to allow her to keep her own Mesorah, but it doesn’t become the default [halachically] when he doesn’t have his own.
If we are talking about personal preference, my point is obviously moot, as is yours in the most case. If we are talking about Halachic precedence, her minhagim would have no more relevance than the next door neighbours.