the thinnest pair of breeches you have and a tank top or a riding top with ventaltion. This might just be me but riding bare back dosent make me sweat as much😂
get some thin breeches or wear breeches that come out in leggings. wear paddock boots instead of tall boots and wear a thin, comfortable, loose t-shirt.
Even though I usually wear breeches and tall boots they make me really hot so skinny jeans and tall boots or paddock boots with a loose tank top is comfy:)