IMHO, potato gratin is best when kept simple!
I use Yukon Gold potatoes sliced thin on the mandolin, a 50/50 blend of 35% cream and buttermilk (just enough to cover the gratin), salt and pepper. Bake covered for 45 minutes at 350 degrees. Then uncover and top with some grated parmesan to help it brown, return to oven for 15 minutes. Then remove it and let rest for at least 20 minutes for the remainder of the cream/buttermilk mix to absorb into the potatoes.
I have found that if you add too much cheese (especially cheddar) in between layers, you get an oily mess.
If you add watery ingredients like ham, kale and onions, this can interfere with the sauce thickening too (caramelized onions are OK though).
Bechamel and butter are not necessary at all.