Hey, I REALLY REALLY REALLY love this game, which is why I'm taking the time to let you know how much I enjoy it and provide some feedback, which is not something I typically do, especially not for fetish games. I'm not sure what your plans are for updates, if any, but I see that it's in beta so hopefully some of this will help you make improvements to the game before calling it complete:
Gameplay:
I think the game is too easy, even on hard mode. Obviously the focus is on being erotic but the pure gameplay on its own is actually surprisingly fun and addictive, the only problem is the game mechanics don't quite mesh the way they are currently implemented. The only fail-state is when the barista's mood falls to zero, but because she gains so much mood from being milked and loses so little from everything else it's incredibly easy to keep her at 100%. Somewhat counterintuitively the game is actually hardest when you first start out because she produces little milk and loses mood quickly from being so small and empty, but once you get a few milk production and max size upgrades in her (which are extremely cheap) and can milk her regularly it's easy to keep her mood at 100% for the rest of the run which removes all sense of pressure and urgency. This means milky mode, which seems to be intended as a challenge mode, actually makes the game easier since you get free milk production boosts. Part of the problem is there is no penalty for milking her whenever you want since you can just reset the customer's order. Customers also wait forever, so you can stop whenever you want and milk the barista to boost her mood as much as you want, throw away the cup of breast-milk, and then complete the order. One fix might be to add a mood and/or cash penalty for resetting a customer's order, so you can't just milk her for free if the customer isn't actually ordering breast milk. For instance, you could lose the cost of the ingredients you threw away in addition to lowering the barista's mood based on how much money you lost. You could also make some customers impatient and give a worse rating if you take too long. I think it would make sense to add changes similar to these to hard-mode to actually make it hard while keeping regular mode more of a sandbox just for learning how the game works / enjoying the fetish material without pressure.
If you did implement these changes it would add a lot more pressure to trying to get through orders quickly so you can use up the barista's milk without wasting money, and her losing mood from having to stay too full while you get through non-breast milk orders might actually become an issue. Then you could add some more fun mechanics, like being able to sneak in a percentage of breast milk in with an order of regular milk in order to try to get rid of it without the customer noticing. Maybe you could also add some upgrades that change the flavor of her milk, so it could be used in place of vanilla creamer, sugar, chocolate syrup, etc.
An idea for more replayability is to give different possible personalities each run. Right now her personality is very simplistic; she really loves being big and getting milked, so once you get a few upgrades into her and keep milking her she basically just becomes lost in ecstasy for the rest of the run and you can't lose. There's no problem with that but I think it would be fun to have some more, harder variations. For instance, a personality that's more shy and loses mood when her nipples are exposed or she's leaking milk, so you have to try to keep her small and covered up unless you're able to get her so big and milk her often enough that the mood gained from the pleasure is more than that lost from embarrassment. Or maybe a personality that really gets annoyed by wasted breast-milk but gains mood seeing customers order and drink it. It'd make multiple playthroughs more interesting while also appealing to more people's specific kinks.
It'd be nice if there were a bit more logic to what people ordered. When you unlock all ingredients people start ordering some truly revolting drinks. I'm ok with the random oddball occasionally showing up and ordering something truly bizarre, but I'd rather most people order sane drinks. Not a big deal by any means I just wanted to mention it as something to think about.
Fetish Content:
This is pretty much just a list of things I'd like to see added for... personal reasons. I'd honestly be willing to pay actual money for most of these.
-More tops like a stretchy t-shirt and tank-top that are able to stretch and keep her covered at much larger sizes than the current tops before finally succumbing and tearing apart. Ideally the shape of her erect nipples would show through, and when she leaked milk it would gush through the fabric and leave big stains. And it'd be perfect if when a white color was selected they became increasingly transparent as the fabric was stretched by her growing breasts, showing the color of her skin, areola, and nipples slightly through the fabric.
-A button-up dress shirt that blows open dramatically.
-A bra
-I know it's not the focus but some ass expansion would be nice, maybe an optional purchase in the cosmetic menu that makes her ass and hips grow whenever you upgrade her max breast size.
-I was really disappointed that thick milk was just a color change and didn't actually adjust the viscosity of her milk. It'd be nice if there were some cosmetics you could buy that actually made the milk particles behave as if the milk was thicker when it's squirting out.
-I'd like more facial expressions for her. An ahegao when she's really being milked for a long time from max capacity, a horny / uncomfortable / trying to endure expression when she's fully maxed out and leaking, an embarrassed / surprised face when her tits pop out of her apron, etc.
-Some more dialogue would be good, she doesn't seem to have many lines and most of them are gags or gameplay hints. It'd be great if she commented more on the feelings of her growth, being filled up with milk, sensitivity, jiggliness, the pleasure of being milked, her thoughts on being ogled by customers, whatever.
-It'd be cool if she could temporarily go even further beyond the 500 cm max under certain conditions, like buying her a fairy latte when she's already maxed or during the increased milk production / dramatic swelling event. Maybe let her hit 600 cm temporarily and then if she's not milked she leaks it out until she's back down to 500.
Anyway, hopefully you continue to work on this. Again, if you polish up the gameplay loop and add some more content along the lines of what I suggested I would definitely buy something like this.