I feel your pain, I really do.
I spoke to my grandmother about this and of all things she suggested personal lubricant… I tried it and it did work, but I’m more a fan of cornstarch powder.
However, when wearing dresses, you can’t always stop and put on some powder wherever you are, but I found a way of stopping it from happening!
I bought some shapewear, the look like a pair of bike shorts, but not as short, the finish about an inch and a half above my knee, so they aren’t noticeable under a dress or skirt. I wore them over stockings (I can’t wear stockings without fear of chafing either) and it was even a warm day and I was out and about for the most part of it and there was not one sign of chafe! I was so pleased. Even if you can’t get ahold of shapewear in that style, I got mine from Kmart for $10-$15, try some bike shorts, they need to be a bit tight so you won’t notice them under a dress or skirt.
I hope this helps!! Good luck! :)