I'm looking to build with a kit using a used Trek 520 or simular for mostly paved road trips / touring / camping. My goal is range, not speed or mountain climbing. 15-17 mph is plenty fast for touring <imo>. I've been riding a 250 front hub Prodecotech ebike for 12 years. It has a 38v 9ah lipo battery that can maybe go 20 miles ( with pedaling). With current affordable technology, what is the most efficient set up to build ?

I live in flat Ohio and understand hills reduce mileage. PS..if you have any vids that discuss efficiency / milage please advise.