Tempting to say buy several 48v 20 ah pings. I would if the requirement was 100 miles at 20 mph. But it's true, to carry 4kw, you don't want gobs of 5 ah cells parallelled and series connected by solder. You want each 2k or so of 3.7v to be a thing you can bolt in when one needs replacing.
I'd say don't get so hung up on the price that you completely ignore practical considerations, such as how do you replace a bad cell. I'd call 20-40 ah hardshell lifepo4 a pretty practical choice. Perhaps a 60v 20- 40 ah pack on each pannier rack?
I'd also advise to not get so hung up on range at 30 mph, that you build yourself an excessively heavy and cumbersome bike. In the west, you may well ride routes that have zero plugs for 100 miles. But most will have something you can get a charge from somewhere every 50-70 miles. I'd build for 60 mile range at 30, and ride it slower on the rare occasions that you can't catch a charge mid ride. I'd carry 40 ah of 60v as an absolute maximum. Why not 48v? because you'd be better served by using a slower winding motor. Such as the pies you own now. So you'd need more than 48v to hit 30 mph I think.
For sure a good longtail can carry amazing weights. That's why I built one, fantasizing I could get away from work for a month of touring NM and CO in summer. But that doesn't mean the longtail won't ride much more comfy with 50 pounds of battery than it will with 100 pounds of battery.
Plus there is a attitude difference with me from many. When I ride a tour on the weekend, I want to ride 20 mph or even less. I've seen these roads in cars at 30-60 mph many many times. It's amazing how I never saw them at all, till I rode them on a bike at 15-20 mph. At 30 mph, I'm just watching the road surface for debris. That's pointless to me.
On tour, the point is not getting there, it's being there. Had the same problem backpacking. Carrying 50 pounds, all you saw of the wilderness around Silverton and Durango was your boots. The cure was to stop trying to get to the trout lake fast. Stop and look every 100 yards, and don't have get there itis.