There's a quite likely reason why he couldn't use a portal to cut Thanos's hand off. He had 5 infinity stones, including the space stone which can manipulate portals with far more elacrity than Dr Strange can. My guess is, unless you caught him totally off guard, he'd just use the space stone to stop the portal from closing.
Of course the movie has built in answers for many questions. Dr Strange said he'd looked at millions of outcomes. He was asked how many they won. He said "only one" It was presented under the pretenses that it was a gloomy outlook. But in fact, it is clear (or if you didn't pick up on it, becomes clear) that Strange was being cryptic. "Only one" wasn't a expression of gloom , but a statement that he'd figured out how to win. In the end, he even says as much when he said "it was the only way" Now, the gauntlet is severely damaged so presumably Thanos can't use it to it's full potential.
Heres rougly how I think it will play out. Thanos actually stepped into Stranges trap. All the other millions upon millions of paths lead to defeat. But Strange said there was one that didnt. We should rember the Anceint One talking about how she had seen the moment of her death long ago, but could see no way past. Presumably she used the Eye of Agemoto the same way. Similary, Strange saw how it all played out. In fact that's one of the most powerful abilities of the time stone. So he chose THAT path. Rember, any path that leaves Thanos alive and the stones in play could lead to Thanos eventually getting them all. In fact, as we see, even if ONLY the time stone remains, and Thanos gets it, he can use that to get the others, even if they are destroyed. And it may not even be in the short term. Any future where Thanos survives and the time stone survives could allow Thanos to complete his plan. Even if it's in hundreds or thousands of years. And Strange could see how all these play out, and probably did well before all of this. (If the Anceint One is any indication, Sorcerers Supreme check their futures to be prepared. Think about it this way, Marvel hero's are usually either lucky or dead. But not the Sorcerer Supreme. It's not luck that the bad guy bets it all on what looks like a sure thing ,but the hero draws an inside straight. Its because any halfway competent Sorcerer, in possession of the Time Stone, cheats. Strange stacked stacked the deck. What he was probably doing on Titan was not seeing what was going to happen, but making sure he had not missed anything. Ironicly, Thanos stepped into the trap and badly underestimated Strange. He chides Strange for not using his most powerful weapon, the Time Stone. It's intended to make you think it was because Strange was keeping it hidden that he didn't use it. But really ,he already HAD used it and was STILL using it , passivly. He followed the path he had determined would lead to victory. As he said, it's in the end game now. And he meant it exactly like a chess game. He'd looked many moves ahead,, setup the board, and now any every move marches toward the inevitable outcome. If you ever watch a pair of Chess Players, more often or not, victory is not when one player captures the others king, it's when one player realizes that ,10 or 20 moves down further, he's going to loose his king, and there's simply no permutation of moves that can stop the inevitable loss. This isn't the first chess analogy from Strange, in the Dr Strange movie, in the end, he wins not by actually defeating his opponnent, but instead by trapping him in a sequence of moves of which there is no way to either escape or claim victory.
The hero's will now regroup. They will likely call on Ant Man and perhaps Hawkeye. They didn't show T'Chalas sister die, so she's still around. Vision appears dead, but he's not. We know this because from a storytelling perspective, it makes no sense whatsoever to kill BOTH Vision and Scarlet Witch. One has to remain to mourn the other. SW is clearly dead, but Vision is an android. Shuri had partially disconnected the his infinity stone and collected a lot of data on him and will probably repair him. Then there's Capt. Marvel, who is reputed to be one of Earth's most powerfully hero's. Her movie comes out just about the time of the next infinity war. They may well intro her just before avengers, and then bring her in as a surprise addition in Infinity War pt 2. Then there's Banner. Banner will probably get his Hulk thing straightened out. He may turn to a scientific solution. It's possible we may see Grey Hulk. Black Panther will return, because he's whoever the new king is. MBaku is a likely candidate. There may be more heart shaped herb growing somewhere, or maybe he'll just get a really advanced suit made with vibranium based tech to offset not having it. Iron Man will return with with Nebula and the Avengers will user her ship to find Thanos. With the gauntlet damaged, they will be able to defeat him and take the stones. Thor is strong enough to weild the stones using the second glove we saw earlier in Nidavellir, using the time stone to undo what Thanos did.
