When I get irritation- usually under my boobs, the best thing I can do is to make sure I gently wash, pat the area dry- do not rub, followed by a hair dryer on cool setting 2 times per day- more if the weather is really hot & sticky. I find it helpful to use flannel to wash & dry the area- its not abrasive, like terry cloth can be...try to get good quality flannel, not the cheap stuff from WalMart that pills after a couple washes. Once I get the area clean & dry, I put Desitin on it- just a thin layer, and I prefer the creamy version personally. I mix a little Tea Tree Oil into my desitin cream- I use the desitin for feeding tube site so I always have it on hand (diaper cream is
a lot cheaper then the stuff my doctors recommended, plus it works better...since they've seen how well it works for me, they've been recommending it to other patients).
In case you don't know what Tea Tree Oil is, its extracted from the leaves of the Tea Tree in Australia and has antibacterial & anti fungal properties- some even say mild deodorant properties too. You can buy 100% pure Tea Tree Oil, though they also sell it at various dilutions, at health food stores and as an ingredient in shampoos, lotions, creams, etc. I had used Tea Tree Oil for several years for minor cuts & scrapes and acne, though I never thought to mix it with the Desitin until I bought some adult diaper cream that had Tea Tree Oil in it- great cream, but considering a tube half the size of baby diaper cream cost 3 times as much, it wasn't worth it for just the Tea Tree Oil, especially when I can add it to the cheaper stuff. When I get a new tube of Desitin, I put about 5 cc Tea Tree Oil in and knead the tube to mix it in- sometimes I find I need to add a little more as the tube gets used. I also mix a little in my shampoo- I feel in love with the Tea Tree Oil shampoo at the salon, but I could never afford it

...now I make my own with whatever shampoo I have
Another big part of getting chronic heat rash & irritations to heal is to expose the area to air at night...I've gotten creative at doing that with my boobs & a tank top...may not look pretty, but it sure helps. With breast, if you're well endowed, the area under your boobs probably hasn't been 100% dry since early puberty

And we know constant moisture isn't good- just like they aren't for Athletes Feet or Jock Itch- both of which as fungal/yeast infections. I guess since
Crease Crud or
Boob Dankness don't have such appealing names for marketing, so we will probably never see any products marketed specifically for it like the guys with their Athlete's Foot & Jock Itch.
You mentioned using powder- personally, it doesn't work for me, if anything, it makes it worse. For one thing, if its corn starch based, the starch can feed the yeastie bugs, if they're present...we don't want well feed, happy yeastie bugs hanging out

Talc based powders shouldn't be used in the genital area, as they have been linked to increase risk of cervical cancer, which doesn't leave any options for the most part. For me, the powder either forms clumps of paste or the individual pieces irritate me the same as a piece of sand can.