My preferred tactic (and a pretty simple one) has always been to back sweeten in the keg with a 1-1.5L of fresh apple juice and then keep it refrigerated. The other kegs just sit around at room temperature until it’s their turn in the fridge, at which point I add juice. This amount of juice just cuts the tartness, but doesn’t make it sweet to taste. If I wanted it much sweeter I’d use frozen apple juice concentrate.
You didn’t mention if you’d be bottling or kegging, so this might not be a very practical reply. I haven’t seen any simple ways to make a back-sweetened cider that doesn’t risk bottle bombs. I guess it depends on how sweet you want it. I’ve heard of people pasteurizing their bottles after back sweetening to taste but haven’t tried it.
Have you had any luck using lactose, stevia or artificial sweeteners? How about bread yeast since it has a lower alcohol tolerance? Not sure what kind of flavour profile it’ll give. Curious to know what others have tried to make sweet ciders.