We know Strange has his sling ring, as their fight on Titan shows Spiderman utilising the portals to attack Thanos.
Correct.. Dr. Strange does have his sling ring when fighting Thanos on Titan, and this is confirmed by Strange manifesting ports for Spider-Man, Star-Lord and Mantis at various times.
Most importantly though, towards the end of the fight there are a few closeups of Dr. Strange, and his sling ring is clearly being worn on his left hand.
Why did Strange not use this to escape, at any time?
Because that would have been anti-plan.
First off, it was completely and entirely possible for Dr. Strange to port both himself and his companions away from Thanos and back to Earth at any point. Doing so, however, would have defeated the purpose of them going to Titan in the first place [instead of heading back to Earth as Strange had initially suggested].
At this point in the film, Thanos already has four of the six infinity stones, and the last two are on Earth (well, one is with Strange, but he was just on Earth). Knowing this, Stark and Strange anticipate Thanos eventually going to Earth, and so, they decide to intercept him on Titan instead, as to avoid killing innocent people and destroying cities.
Even after their failed attempt in removing the gauntlet, and despite Thanos hurdling a moon at Strange/Stark/Spider-Man/etc., all of them still stay around and keep attacking Thanos. In fact, there's never a comment from anyone about wanting to leave. That being said, none of them wanted to leave, and porting back to Earth (or anywhere else) wasn't even a consideration.
Even before Strange looked into the future (and possibly accepted his fate), there was a large plot point about Stark/Parker/Strange being trapped on Thanos' Qship, with no means of escape, and being forced to make a crash landing. Surely the sling-ring should make this a needless risk?
They were never trapped, per se, and the crash landing was impromptu.
After Stark and Parker save Dr. Strange, they immediately have a conversation about how to proceed. Initially, Dr. Strange proposes that they turn the ship around and head back to Earth; Stark on the other hand suggests to let the ship continue its course to Titan and fight Thanos there.
There was a moment when Dr. Strange asked Stark if he could fly the ship, but Stark neither confirmed nor denied that he could. Instead of answering Strange's question, Tony responds by proposing his own plan.
And then, it was only once they began entering Titan's atmosphere that the ship started losing stability/integrity, at which point Stark and Parker together attempt to manually steer, only to crash into stuff along the way down. This is what caused the ship to be in such dire condition upon landing.
Since Dr. Strange has never been to Titan, they had to take the ship to get there, which included enduring the rough landing.
There is even a scene earlier in the movie wherein Black Dwarf has his arm cut off below the elbow by strategic use of a sling-ring portal. Considering their entire end game is to get Thanos' gauntlet off his arm, this would have been a good repeat strategy!
This port was created by Wong, not Strange, so there's no reason for Strange to think of it when fighting Thanos (w.r.t. the previous event with Black Dwarf). Also, Wong was just closing the port after Black Dwarf had already gone through it, and not timing the port to intentionally slice his arm off (Black Dwarf was reaching forward when running back to the port).
It does sound like a good strategy though, since all they have to do is remove the gauntlet (i.e., chop off his arm)..
His sling ring seems heavily under-utilised; is there any stated reason for this?
IMO, Dr. Strange's sling ring usage is overall well balanced, and well used.
