If the congress rejects this deal, it cripples the US ability to negotiate in the future and essentially just forces an unconditional US surrender as nobody has appetite for a ground invasion of Iran.

Congress can only sabotage deals like this at an immense cost to the US’s future ability to negotiate anything with anyone, and it certainly can’t sabotage it’s way into a more favourable deal.

I think you're underestimating both the amount of damage Trump has already done for the future ability to negotiate, and the desire of everyone else to be not bombed by the US.

Everyone already knows, any deal with the US does not bind the US, only the other party. But it still might be preferrable to the alternative of no deal at all.