Does a glove exist that has grippy stuff on the thumb so that once you completely saturate your gloves with sweat you can still shift? USWE has provided enough grip on the palms I don’t slide off my handlebars but I still can’t shift or use my dropper with any reliability on these hot/humid long rides.
As a super heavy sweater I’ve never had this issue. Have you tried adding anti-slip tape to the shifters? Also, the Woolftooth dropper lever (and I’m sure others) have a textured surface, that should help.
Wow. Put something sticky on the shifters. Brilliant. Embarrassed it never dawned on me to approach it that way 
Just wondering if it’s worth wearing wristbands to stop arm sweat running down into your gloves, as well as the hand sweat that’s already there. Nice to get a bit of air current across your wrists though!
You don’t mention braking but if that’s an issue Lizard Skins have lever grips:
For your shifters, I knew some people back in the day that used Skateboard grip tape. It’s sticky on one side and like sandpaper on the other.