seem like solid choices on the lower end of the price range. My concerns with extreme back injuries is if you can really peddle and sit on a normal bike without pain while peddling too. It's a fairly uncomfortable thing for most healthy people, so you'll have to be careful.
Have you gotten on a regular bike to see if you can ride without pain? The next day? On a regular bike, potholes and other frequent bumps can really annoy some folks backs. I would definitely ride a regular bike for a week or two to make sure you're bodies ok with it before spending a lot of money on an Ebike.
Personally, I'd be looking at full suspension bikes considering your back. That likely puts you into buying used, more expensive bikes. My Haibike got me back (full time) on uprgiht bikes Because of the suspension. I can't ride a rigid upright due to my carpal tunnel syndrom (hands) and tender tushy! but after some time building up my tolerence I can now ride 20 miles on it. That's after 15 years of recumbent only due to those problems.
Nigh on amazing to me how much it helps. You could buy front suspension only (much cheaper) and add a floatation seat post for the rear. Thudbuster or Bodyfloat.
Truthfully for back issues, after 20 years experience and endless hours on bentrideronline.com, a trike simply can't be touched for comfort and many rave about how it HELPS their back.
There's something about that position (feet out front, back supported and completely relaxed) and the gentle peddling motion that you actually get on and ride the trike to EASE your back pain.. Almost any time I get on it with some back stiffness/pain, I get off it feeling much better.
If possible I would highly recommend you try one! Mine has a 500w Falco hub motor in the rear wheel. Love it but they are on the expensive (but worth it) end of the scale. FWIW
