This is my first Solstice, but the guides say you get more ash-per-leaf conversion ratio the more balls you dunk in the fire (20 max?). So if you stick together, everyone gets a ball to dunk faster than you doing one champ solo and trying to run back to get a second ball before it times out and disappears, depending. At least that seemed to be how it felt during my bonfire run...
Max is 30 this year. Assuming you have at least 20 silver leaves and max ignitions you’ll net 20 ash per piece of solstice gear equipped. So if you have all 5 on you’ll get 100 silver ash. You can just equip all 5 pieces once the final boss spawns in, you don’t have to have them equipped.
Sticking together is generally the best strategy, especially playing with randoms. Once you get the throws down there’s only two spots where you can’t throw the ignition charges from where they drop (the alleys to the left and right of where the boss spawns) so when I’m playing with my friends we’ll sometimes split up a bit and I’ll clear an area by myself when I know I can throw all 3 in myself before rejoining them
I've only played a few runs so far, but I really like that there's a reason to stick together this time around. Clearing waves of enemies isn't too much of an issue (hint: just equip Graviton Lance if you're having trouble), but some of the spawn locations are annoying to throw from.