Personally, I can’t stand mummy bags. My feet stayed “tied up” all night.
For the conditions you are talking about where weight isn’t a factor, I take two bags..a medium weight one inside a heavier bag and a couple of wool blankets just in case. If it comes down to using the blanket(s) I open them out, put the bags on top and then pull the blankets over the bags.
You can always remove layers but you can’t add any if you don’t have them available.
