i have two eBikes. one 500w and one 300w (actual max power consumption). 45lb and 28lb. my riding is very similar to yours - get out and see the countryside and burn calories, but we have a LOT of hills here. guess which one i ride 99% of the time that i’m riding for pleasure/exercise?
seriously, the comments about needing 1000w or something are from people who are using their bikes differently. for the kind of riding you describe, first priority is keeping the bike natural, fun, light, nimble, and inspiring.
unless you’re a competitive level cyclist, your average output is likely in the 150-250 watt range. the smallest motor you could get is capable of more than doubling your output. i’m 6’2, 185lb, and can sustain around 200w indefinitely, 250-300 for 5-10 minutes. my 300w mid drive makes me fearless on a long ride - i leave it off 95% of the time but if a huge hill, crazy headwind, or some health issue comes along i can crank it up. turned off, there is no drag and it’s almost as aerodynamic as a traditional endurance geometry road bike. the big downside is lugging an extra (relative to a light climbing road bike) 10lb of bike up the hills when i have the motor off.
my advice - you’ll like having a motor and battery to extend your range and open up new routes, perhaps ones with hills. if you’re going to do short (<30 miles) rides without much climbing, all the motor will do is boost your average speed by 5mph or so and make your bike heavier. (doubling your power from 200 to 400 watts at 19.5 mph is good for 25mph….) it’s critical that the bike still ride like a bike with the motor off or on a very low setting, and critical that you can tune the output based on speed and torque.