White ash works good, a lot of old barns around here have ash in the lofts (which in other areas folks call hay mow)
But if you can find cottonwood, I would highly recommend using it for the top layer.
The advantage of cottonwood is that it is 'spongy' -the fibers in the wood will compress and crush under extreme loads, but will spring back to their normal state. This makes the wood incredibly tough and durable. Old timers made their floors out of cottonwood because it could take a serious beating.
Ash under a crushing load will be crushed, and the fibers destroyed.

I am a big fan also of like you said supporting your floor with a mid-span post.
I am not sure exactly what you are describing, but what I would do is have posts to support a beam upon which the joists merely set, they do not have to be joined at all, here gravity can do ALL the work.
I know this idea works very well, and I can show you examples of castle floors framed in the 12th and 13th centuries like this that are still functioning well to this day under heavy loads.