The rule for me is something you wouldn’t mind flashing accidentally, that creates a thigh friction barrier.
I (40+ cis woman) personally like the legging material kind because they’re plentiful and I’m lazy and thrifty. I wear regular underpants under them most of the time, but not always. These work under any length skirt.
I also wear ruffly light linen shorts under loose dresses/skirts. Or outerwear shorts, as long as the inseam length is long enough to block thigh rub. True spandex bicycle shorts are beyond my ken, but they seem to work for bicycle people.
And I’ve never shopped them, but online underwear stores like woxer or meundies have super cute designs and thigh length panties.
Happy summer!
During heatwaves, when I change after work, I’ll powder myself pits to knees in a dry shower, then get dressed. The dust gets everywhere.