I live in the suburban Philadelphia area and there does not seem to be a lot of bike shops that are nearby but there are a few. The shops that are nearby seem to carry trek bicycles, That seem to be a bit more expensive and seem to be equipped with motors and batteries that are of lower wattage land some of the online companies The trek verve 3+ which seem to fit my needs but comes with a 500 W battery rather than 750
You're talking about different type of motor. Mid-drive (like on Verve) is low 250W but it climbs hills "almost" same well as 500W geared hub and much better than direct drive hub. It uses less energy, so 400WH battery is adequate. 500W DD hub will climb too, when climb isn't too long. But you probably don't need Verve in your scenario. Philly is mostly flat, very moderate hills as I recall from a brief visit a while ago, though it was not in suburbs. Big brand mid-drives like Bosch used on Trek and many others have no throttle (this partially adds to"fuel economy" of this type of motor).
If you decide that you want a mid-drive without paying too much, you may look into Biktrix Stunner. Easy IGH gear shifting - sort of an automatic "transmission" on a bike. Unlike Bosch motors, this one has a throttle. Similarly to RAD it is online only.
As to the initial assembling of online-only bike, I'm pretty sure that your area is served by Velofix mobile service, they will assemble any bike and will help servicing it later. I think with RAD they just receive it for you and deliver already assembled and tuned. With other bikes you receive the box and call Velofix, they pick it up from you and bring it back assembled. Talk to them.
At this age many prefer "step through" frames, this is something not to be ignored.
"Comfortable/uncomfortable"... Many replace the stock saddle, if this is causing problems. There are also upright ridden bikes VS lean-forward models, different type of handlebars on those. If you're not a long-range commuter and not into serious trails, chances are that you need more upright position. There is also foot-forward feature on some bikes (Electra Townie Go and Aventon, for example) - seat tube is shorter but pedals are farther forward so you are sitting like on a chair, legs above the knee are close to horizontal.
Terminology detail - motor power is measured specked in watts, battery capacity - in watt hours.