Who to fodder special spiral to

This is a bit niche, but you can also have a Saber with Blue Flame and his refine with Special Spiral. Some other ideas could include Flashing Blade Lewyn with a blazing AOE special and/or Ophelia with a blazing AOE special

why not reinhardt? activating Luna every battle?

My dream is Sonya, Ophelia, and Lilina. There's also a Brave Bow setup I use with bonfire. I don't particularly like any of the options you gave for it because I can think of other options they'd want more. 100% not BK at the very least