I have left thumb issues (pain) when shifting on a flat bar mtb with triggers. I went with an old Suntour XCD 6000. It is so enormous I can shift with my palm. It is friction and you'll have to modify the clamp for road bars, but it has eliminated using my thumb for shifting. FYI, Grip Shift didn't work.
https://velobase.com/ViewComponent.a...df184&Enum=104
John