I don't know what or who you ride with, ie what if anything is on the passenger seat.
I ride solo and have a Cortech "soft" bag attached to the passenger seat.
It was tricky to fit but works well.
As a test I made up a "cushion" to attach to the front edge of the bag which pushes against my lower back. You could try different sizes of foam to see what might work.
To be honest I haven't actually tried it out yet on the road (only in the garage where it seemed quite comfortable), but may do soon if the weather clears.
I think most lower back pain is caused by bending the lower part of the spine in a semi-permanent fashion when we ride. ie we tend to stretch forward to the bars, which as anyone who has ridden sports bikes as I have for many years would know, it causes "discomfort" and semi numbness in the lower back and shoulders, arms and hands.
The more upright you ride and the more relaxed the arms the more comfortable you are.
Personally I find the dino quite a comfortable setup with the boards, swept back bars and roomy seat.
I had a couple of bikes to which I fitted backrests and found them to be too inconvenient getting on and off and I could never quite get them in the sweet spot.
Another option might be to try a "kidney/weight lifting belt when you ride as they help to keep your back straight.
Anyways, a couple of low cost alternatives to try before lashing out hundreds of dollars on an expensive "maybe" seat
good luck with your search
Pete