Sram shifters shift both up and down with the thumb and I've not tried a front Shimano shifter but at least on the rear Shimano trigger shifter I've used, the paddle to shift up a gear can be pulled with the finger or pushed with the thumb for the same result.
Good to know re: SRAM - I’ll investigate. Shimano is a no-go based on trying out my partner’s bike with a front mech… adding work beyond just braking triggers all sorts of interesting aches (lacking a usable index finger means the others have to work harder than they seem to want to in general).
Sram shifters shift both up and down with the thumb and I've not tried a front Shimano shifter but at least on the rear Shimano trigger shifter I've used, the paddle to shift up a gear can be pulled with the finger or pushed with the thumb for the same result.