2x6's aren't conventional for floor structure. I would never use 2x6 in a floor close to the ground.
However, 2x6's can actually go a little over 9 feet span (from the book, Building Construction Illustrated). Your deck thickness is very strong as is.
I would level the sagging areas with a sistered piece, 2x6 or plywood. I may even do every other one glued and screwed. You want a stronger than normal floor for large tile.
