Hey @ADivineCanine !
I lived in the deep south for almost a decade...so I know how important it is to have a plan in place for hot and humid weather!
I only wear synthetic underwear with flat seams when doing any kind of outdoor activity. I also like the Ex Officio underwear. My usual go-to are these Fruit of the Loom panties which we don't carry at REI.
In the summer, I pretty much only run in compression shorts, which helps both with moisture management and chafing. I'll also wear them under dresses and skirts for the same reason. If you have a tight enough pair, you could easily wear them under your current pants. There could be a potential that they might make you hotter; for me, they seem to create enough of a barrier that prevents sweat from showing through. I prefer wearing Nike Pro Compression Shorts 5", but they also make shorts as short as 3" and as long as 8".
Finally, you can also try switching to a dark pant that's made with a moisture-wicking material. The PrAna Halle pant is made from a wicking material called "Stretch Zion." I own a different pant from PrAna made out of the same material in all three colors, and have not seen much sweat through them.
Hopefully all these ideas help you get ready for an active summer!