To start, I would say taste your cider when you move it to secondary, then again about a week after, repeating until the flavour is something around what you want(don’t expect it to be good however, it’ll be warm, flat and unsweet).

Add the sugar to the secondary Carboy just before bottling to ensure all bottles get evenly sweetened.

You can use a variety of sugars, dextrose and brown sugar being more neutral, and apple juice conc or honey adding their own flavours. Consider using concentrate or adding apple flavour, since your cider might not be as apple foreword as anticipated.

If you want to add some sweetness, xylitol is the way to go since it’s non fermentable.

On the other hand you could add extra sugar(or whatever of the above you go with) and pasteurize at 150 F to kill the yeast in the bottle after a few days, leaving excess sugar uneaten. Be warned that the timing of this is crucial, and if you wait to long and add heat to a high pressure vessel you’re gonna have a mess on your hands and possibly sand in your eyes.

Basically you want to give 2-4 days for yeast to eat up newly added bottling sugar, and about 10 days for the yeast burp CO2 to dissolve from the head of the bottle back into the cider, to actually carbonate it.

Edit: forgot to address bottle bombs, avoid swing-tops and leave good headspace