First question is if you're bottling or kegging? You have a few more options if you're kegging.
If you're bottling then you can sweeten with maltodextrin which isn't fermentable - I think that would be better than trying to steep crystal malt. The maltodextrin sweetness doesn't last forever, but it will last 6 months.
If you keg, then sulfite and sorbate will stop the yeast. Then you can sweeten to taste.
Or you can stop the yeast, sweeten and bottle it still. But most people like carbonated cider.