Only you know what your specific back situation is, but I can't emphasize how wider and wider tires make such a difference to more comfort for hands and bum/back.
I've ridden on 28, 32,35, 40, 50, 55 and it's been very interesting noticing the changes and appreciating the increasing cush. Of course some tires are more supple so they h have a nicer feel and "give" , which your n back would really appreciate.
anyway, some thought to a bike that could take much wider than 28, it could very well make all the difference for your comfort.