Welcome, Bob.
First problem you have is the joist structure. If you knew the grade and species of your joists it's possible they meet the deflection requirement for a natural stone tile installation, but it's not something you want to guess at. And you do understand that you need to know the unsupported span of the joists, which is not necessarily related to the size of the room to be tiled, yes?
Second is the subfloor. The stone industry requires a double layer of plywood subflooring regardless the joist spacing. And that applies regardless the size of your tiles. I personally believe all mosaic tiles should be treated as natural stone installations on floors, but that's another discussion.
Your problem here is that you don't have a first layer of subflooring. That nominal 1/2" plywood does not have T&G edges and is too thin to even meet building code if it had blocking under the edges, which it does not. Seriously, Bob, it just doesn't even count. The absolute minimum you can use for the first layer is nominal 5/8ths" plywood with a second layer of a minimum of nominal 1/2" plywood properly, and differently, installed over that. Then the tiling substrate of your choice.
You need to make your floor suitable for your tile installation, then make whatever transitions necessary to accommodate adjacent flooring.
My opinion; worth price charged.
