I definitely disagree that it wouldn't have fit with the downer ending. The biggest possible "downer" effect would come from a crushing blow delivered right after a climactic high - When the wrong person snatches victory from the jaws of defeat. Thanos snapped right when it seemed like our heroes had the cat in the bag - this wouldn't have changed just because Smart-Hulk had a break through earlier in the fight.
But it wouldn't fit. Smart Hulk is way too much of a shocking and funny revelation, it would've gotten in the way of the sad ending. Imagine seeing Smart Hulk standing there at the end instead of the Hulkbuster, after only 'knowing' him for one fight scene previously. The absurdity of Smart Hulk would stick out like a sore thumb and wouldn't get time to breathe at all. That stuff really wouldn't work. When you see him first in Endgame, they take their time to get all the laughs out of just seeing this strange merging. And even then, next scene you see him in (testing time travel), he's still funny just by virtue of being Smart Hulk. It takes quite some time for Smart Hulk to get some actual serious scenes, and understandably so.
Strongly disagree that we don't get to see Thor's struggle. Thor's entire Endgame arc is about his ongoing struggle.
Sure, my point was we timeskip ahead just like with Banner, to see the outcome of how they deal with their failure. We see Thor at a point where he's already gone way off rails. Just like we see Smart Hulk at a point where's gone way on rails, or however you want to phrase it.
I'm contrast, the Banner/Hulk arc that begins in IW, ends offscreen DURING the time skip. Also, I didn't feel like it was presented as Banner/Hulk dealing with failure- it was the two of the dealing with Co-existence
It was explicitly presented as Banner/Hulk dealing with failure, he says that during the first scene we see him in in Endgame: first the Hulk lost, then Banner lost. They use that loss as a motivator to solve their differences.
Also, even though I agree Smart Hulk has less of an arc during Endgame, his arc isn't finished when we first see him in Endgame. His snap ends it, I'd say. But yeah, it's certainly less of a journey than Thor has.