At 6'4" and 235 pounds, you aren't all that heavy, even though you may be heavier than you're used to being. I'd say you're doing the right stuff so far; namely, getting test rides. The thing with backs is, they're all different. The seat that feels great to one person with back problems, might be agony for the next. Keep looking for opportunities to test-ride trikes - both delta and tadpole if possible. The one problem you might have with a tad is the seat height and whether or not your back will let you get down into and out of it. Don't be afraid to try those 2-wheelers again, either. One tidbit I like to suggest is to repeat the test rides a week later. It takes your brain a few days to process the first experience and you might find even the one you hated before has become kinda nice. Or not. If the one you didn't like was an EZ-Speedster, I don't like it either.