I've noticed if you take to long whilst doing it in milky mode the happiness drops like a lead balloon from overfill, I'm trying to handle it with over 2 million ml production, its better to get bad drinks made by just adding the accessory items (whipped cream, whatever is req for whatever drink) and just click until all but a tiny sliver of the req is gold so you can quickly send it out then immediately clock and hold for like the next 2 mins
edit, also the "no milk production event" does literally nothing once you reach 2 mil, it still fills immediately
edit2, milk flooding is a thing and its game over, so this is way way harder then i anticipated, that last run was like 40 mins
edit3, I've noticed its way easier (but also takes way longer) if you stay with the basic everything (but upgrades) for a while to try to max out happiness and get some beginning cash
edit4, might just be me, but if you completely lower texture quality its a lil easier
edit5, ok, if possible try to avoid getting over 3mil ml production, it starts to increase by the millions after that
edit6, ok, once you pass 60k production, the no milk event counts like you only have like maybe 500 production, so my guess is it wasn't coded to set the production to 0, but it was programmed to subtract it by about maybe 55k and have it incapable of going under 0 (well, mostly, if you lag out the game it can sometimes jitter between -0.8 and 0, its interesting)
edit7, fixed grammar
edit8, it is possible, just immensely difficult, you really need to be as efficient as you can at all times, especially near the beginning. i was only 20 dollars away from buying the final upgrade, I'm going to test if it is actually easier without milky
edit9, it seems like if you never make the initial purchase of the candy, then she never starts lactating, you can make as much cash as you need beforehand with simple drinks then just quickly spend it all
edit10, with the headrubbing you just barely outpace any decreasing from events so as long as you are quick and accurate it is so much easier without milky, milky is almost definitely hypothetically faster however. (if someone ever decides to speedrun this game, lol)
edit11, I am genuinely surprised anyone has completed this already, by my estimation it is going to take at least 3 more hours to complete it (maybe I'm just doing it highly inefficiently)