Loved the story right up until this bit

..when Carl symbolically accepts the past by ridding his house of its Ellie-related contents in order to make it airborne.

That was the main battle in the film, between his past and present. After that the battle with Muntz's gang was merely a physical one, with no symbolic stakes. I'm not against action, but when it becomes merely a sequence of stunts with no meaning, I immediately get bored.

Don't get me wrong, I loved the poignancy of Carl's backstory and the adventure's meaning. If somehow the acceptance of past had been timed with the physical battle, everything would have come together, similar to the Carl's previous choice of saving the burning house at the cost of Muntz capturing the netted Snape. In other words, they needed to delay resolving the emotional stakes until the end of the film.