My wife has had back trouble for years. She rides a bike with "sit-up-and-beg" handlebars and her back is nearly vertical. Fairly wide Schwalbe Supreme 42 mm tyres help with the comfort, they still roll well even with quite low pressures. It seems to work for her, so maybe you should try and borrow a bike like that to see if a radical change in position works for you or not. For a test ride, it doesn't have to be an E-bike.
I've ridden that style of bike fairly often myself, notably borrowed bikes in the Netherlands and the "visitor" bikes I keep at our island flat. "Sit-up-and-beg" isn't what I prefer (I may change my mind when I get older), but with a suitable wide saddle (say a Brooks with springs) I am comfortable enough, just a bit slower due to the increased air resistance.