Well, when I was new to this whole thing I wrote in this one so allow me to share what I know. Here's what the audience wants and how to give it to them.
Covers: Boobs. Bigger the better, just make sure not to get filtered. Speaking of which, it may seem like a good idea to make a cover image with a lady drinking a glass of milk or something but Amazon sometimes filters that (especially if it's a milk stain on the lady's lips, it can look like jizz) so maybe don't. If you could find a busty gal holding a milk carton then you're in business. Other than that some good signals of what your story is about would be: breasts, cow hide print, busty farmer girls, etc.
Blurb: As stated, lactation and milk are no-nos. Get creative. Creamy goodness, she feels "full, bursting, and craves release" and so on.
Reader Expectations: This is kind of a two-parter. There are two camps in this niche: lactation and hucow. Lactation obviously comes up prominently in hucow stuff, but not always the other way around. There is erotica out there of just a lactating lady getting it on with someone, but that doesn't make it hucow erotica. Reader expectations for lactation would be her partner drinking the milk, the release/pleasure she gets from that, lots of focus on the breasts (looking plump, ample, full, swollen) and lots of focus on the nipples. For hucow things get a bit heavier. A very popular trope (though not always an expectation) is there being a farm of hucows and the FMC stumbles upon it, applies for a job there, etc, and experiences it. A big expectation is a transformation from human to hucow. Usually some kind of pill, potion, whatever, that makes the FMC's breasts bigger and causes them to lactate. If it's a farm setting the milk is almost always pumped mechanically, and usually orgasm causes her to produce more milk. There's usually a farmer or farmhands or cowboys who work on the farm and give FMC said orgasm. There also might be "bulls" on the farm to breed with the hucows, but this varies by story.
Side Kinks: There's often some level of bondage with hucow stuff, in that the FMC is often strapped into something while being milked. Breast torture (clamps, bondage) is sometimes included but not always and some readers don't dig it. Pregnancy gets used a lot with both lactation and hucow stuff. If it's a farm hucow story then there's usually some kind of D/s relationship between the farmer/cowboy and the FMC in that they treat her like an animal or property.
Length: Readers tend to like these shorter (under 10k and usually closer to 5k). This stuff is pretty much pure wank, they aren't interested in novellas. Series do well, though. Keeping it on the same farm with different FMCs being introduced, or one FMC experiencing different things on the farm in different stories.
Profitability: Meh. Dedicated audience but low ceiling. You won't make it big writing this stuff but if you enjoy it then it can serve as pretty steady income. I was making around $300 a month off a few 5k stories a week in this and I still get sales on them. Good place to start for beer money but if you want to make this a full-time gig then you'll need to look elsewhere. There just isn't a big enough audience to get high earnings.