Welcome, Jess.
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The 3/8ths" porcelain will be quite adequate for your application if properly installed over a proper substrate.
It's the substrate that worries me most. A photo or two of what you're working with would certainly be helpful. It sounds like you're ending up with a whole lot of construction joints that will need to be honored through the tile surface.
What size, shape, layout do you have in mind for the tiles?
My opinion; worth price charged.