As you note, Superman is hit by Iron Man's blast - HE IS STILL STANDING.
Hercules, Iron Man, Vision and Wonder Man join the frey and I read that as desperation. If my friend Rob has just been ko'd by some guy in a club and Stuart punches him in the face and he still doesn't go down, until he is joined by myself and 3 other blokes, pretty sure it means it took all of us to knock him out. It is in fact an embarrassment that it took so many of them.
Whilst all this was happening and Thor was recuperating, Superman was taking additional punishment. Even they Superman took no longer than Thor to get up.
So Superman weathered all Thor could give, all that half a dozen Avengers could give. During that time Thor was recovering from Superman's attack.
If you want to use additional comics, then I would say Doomsday vs Superman, where he got killed was a tougher fight than Thor.
I know you don't want to hear it, but Thor is no match for Superman.
Notice that Superman is hit with IM’s blast, punched by WM and then bopped on top of the head by IM. We then see him going down from the attack of just those two. Three panels that would have taken three seconds at most. The others don’t reach him until he is down. They are saying “you don’t do that to him”. That’s payback, not desperation. They are hitting him while he is on the ground.
When we next see Superman and Thor, Superman is on his hand and knees being helped up. Thor is sitting, talking to Wanda. Superman is clearly in the process of rising from the prone position he was in. Thor is in a settled seat that he probably had been in for a while.
Superman believed that Thor may have been his toughest opponent. So clearly he wasn’t sure if Doomsday was tougher.