Getting him off the island is not hard if he's reluctantly, but willing, to go with Rey instead of being a coward like they made him out to be. Now I know why Hamill had such issues with the way they wrote him.
You don't think physically being there with the green light Saber, the one Kylo saw his life flash before his eyes with, And taking thinking he's going to take Luke out, only to realize Luke gave himself up to the force, would have been and even bigger mind job on Kylo? And that's after surviving the At-At assault.