I think, after a while, you'll find yourself using your watt meter to monitor that motor. It's very easy to see how much load is on it at any given time. Keep that minimized to maximize mileage between charges, and monitor it to help select the right gear and PAS level. Lot's of watts in use show potential lugging. If you aren't in a big hurry, try dropping a couple of gears or a PAS level to see how that compares.
Electric motors have a huge (wide) power band they are comfortable operating in (as mentioned, check out transmissions used in electric cars - there isn't one as there is no need for it). They are nothing like an internal combustion engine that needs to be kept within a narrow power band to run efficiently maximize power. In fact, most motors we'll see in these bikes are making max power (torque) from the time they leave a stop sign....
Too, selecting the "right" gear is mostly about your preferred cadence level. The "right" gear is the gear you use to maintain your preferred cadence level. The motor doesn't really care what that is (it's watching your PAS level mostly). When you are motoring along all fat, dumb, and happy and approach a hill for instance, it's going to become difficult to maintain your favorite cadence. So you have some choices to make. You can increase your PAS level to ask for some extra help from the motor, or you can drop a couple of gears to let the motor continue "as-is" and YOU supply the necessary power to climb through a gear reduction, maintaining that preferred cadence level.
All this is a lot to think about at first. Most of us, myself included, totally sucked when it came to riding an e-bike efficiently early on. After a while, things start making more sense, and by time you get a couple hundred miles on, things will have a lot more focus as you build some experience.
My first e-bike was about 2015. To this day I play a game with myself. It's about how many miles I can get on a charge.... That's the easiest way to judge how efficiently you are riding.