Top left of the Difficulty selection screen, there is a lighter colored circle, that is this so called "hidden button" for Chaos mode
Ironically I found the Milky Mode to be easier for Ultra Chaos, than doing that difficulty without. Simply because it gives some extra breast milk production allowing you to get breast milk added to the otherwise many pure milk orders from the very start. For non milky mode, really a lucky seed that allow you to do some cofee included orders at the very start, to get both the cheapest toppings early, and some production and capacity upgrades so you again can get breast milk into the orders as early as possible. For both with and without milkyy eventually getting all the toppings, and otherwise just focus on enough productions(unless on milky mode ofc) to always have breastmilk, some fullness tolerance and just rush the size then. I don't even get any other fillings carrying it all on toppings+breastmilk really.
That is at least what works for me