In my experience the issue is one of torque, not of power. Most cheaper electric bicycles (including most Decathlon models) have a hub-mounted motor which cannot deliver power effectively at low speeds -> it stops assisting when you most need it. For hilly terrain you need a motor that's mounted on the crankshaft so that it can take advantage of the gears.
To compare: a tour de france runner outputs 400-500W on mountain climbs while keeping up to 20-25km/h. So a 500W motor (plus 100W power output from yourself) should be enough for smaller climbs!
To compare: a tour de france runner outputs 400-500W on mountain climbs while keeping up to 20-25km/h. So a 500W motor (plus 100W power output from yourself) should be enough for smaller climbs!