There are a few thing missing I'm afraid. Thor is "not captured" by Surtur. He dreamt Surtur was the one responsible for Ragnarok and lets himself get captured so he can come close to him and stop/kill him to try and prevent Ragnarok (as for Surtur not being giant, just watch the movie and you will see). And he and Hela do not "go trough the ground together". Surtur makes a big chunk of rocks and buldings fall on Hela, maybe Killing her. With her out of the way, he procedes to destroying Asgard as it had been foretold.
As for the fight between Thor and Hulk it is true that Thor loses but there is a little thing missing on your description... Thor IS winning after he learns how to use he's powers and Hulk is almost about to lose when Grandmaster intervenes. So, yes, Grandmaster did so because he saw Hulk was going to get beaten and koed. About Hulk and Surtur... I don't understand Thor's worries. I mean clearly the Hulk standed no chance against Surtur. When he jumps at him Surtur just throws him away like a bug and procedes to destroying Asgard. He doesn't even care for Hulk or the others being there. There's one thing Surtur also said to Thor before they sart fighting at the beginning of the movie of the movie. Like Hela, being close or in Asgard, his powers reach their maximum, with Surtur showing that while in Asgard he's more powerfull than even Odin (something to do with Surtur being some kind of Asgardian as well).
I liked the movie but there was still something missing for it to be great.