There is a guy around here with the same problem. He rides almost every day.
I saw him yesterday.
The solution is very simple and easy. Skip derailleurs, the end result is not that good.
The LBS set him up with an internally geared coaster brake hub bike.
It could also cost more to modify your bike than buy a new bike with an internally geared coaster brake hub.
Rear brake...backpedal.
Front brake..left hand
Shift ..turn left grip.
He has an artificial right hand that looks like it is holding a stick. The LBS put a short straight bar end on the right grip for him to put the artificial hand over.
There are rear hubs with 8 and 14 speeds. Not sure if the Rolhoff 14 speed hub comes in coaster brake. Try googling it. If you want to get on the road soon and keep it simple do this.