I love stouts.
I would shoot for a sac rest around 154.
A tiny amount of Vanilla extract post fermentation will help ALOT with making it seem sweeter then it is. Vanilla is very difficult to judge. Best bet is to add a small amount and let it sit for 24 hours and then try it again and add more if needed. I add 250ml into a 76 gal starting volume batch and it is noticeable.
that is .05ml/50ml approximately. But i would half that initially and give it time to blend into the overall flavor.
I would add a heavy dosing of oats. heavy on the chocolate malt and some dextrin malt would not hurt as it can leave some residual sweetness. Golden Promise base malt can also help impart some extra sweetness.
I frequently use debittered black malts and midnight wheat to add color without extra bitterness.
This is my marshmallow stout and it is sweet and really decadent. You could get rid of the marshmallow flavoring and it would fit the bill imo.
https://www.brewersfriend.com/homebrew/recipe/embed/1412778