My mother has always used cream cheese as a filling ingredient. Those things are dense.
Also, don’t lose the opportunity for a tasty shell. Oil the skin and season with a course salt and whatever else you like.  But I would not experiment on a rehearsal party.

My best was made with some leftover red potatoes leftover from a potato salad cook. Mixed some in with the regular mash with a dash of cream cheese, butter, a bit of Liptons Onion Soup mix. Just a light sprinkle of cheddar and chives on the top.
Had sour cream on the side, but nobody used it.