Well, you can definitely rule out Honda sedans.
Although my '92 Accord was a relatively decent car overall, the extremely low placement of the seat led to a “legs out” driving position that led to severe lower back pain and leg numbness on a drive longer than 30 minutes or so. When driving newer Hondas belonging to friends, I found that the seats are still placed lower than on any other sedans that I have driven, and that they still produce the same back/leg problems for me.
All of that being said, the best bet for your needs would be one of the smaller Crossover/SUVs, like the Honda CR-V, the Toyota RAV-4, or the Subaru Forester. The seats in those cars are exactly “butt height” for most people, and that means you can literally just slide onto and off of those seats, with no lowering necessary when entering, and no rising necessary when exiting.
I find that the seats in the RAV-4 are decent, but when I test-drove a Forester, I found that the lumbar support was inadequate (for me) and thus caused back pain for me after only ~15 minutes. I can’t comment on the seat comfort in the CR-V.
However, since seat comfort is such an individual thing, you really need to take an extended test drive in all of these vehicles in order to see what is comfortable for you. Tell the salesperson of your back problems, and inform him/her that you need to be able to do a test-drive of ~ 1 hour. Most dealerships will be okay with this, as long as you tell them ahead of time.
As to fuel economy and cargo room, these vehicles will all deliver overall gas mileage of ~23 mpg, and have surprisingly spacious cargo areas.