Flooring Tile - Uneven subfloor - higher at joist

I'm planning to lay down porcelain tile in our 3.4 x 8.6 entrance way. I removed a layer of gypcrete to reveal a plywood subfloor. I plan to lay down Durock followed by the tile, using Mapei Keraset thinset with a Keraply additive.

The floor is slightly higher in the center, where two pieces of plywood butt together (I assume over a joist?). If I balance a 4 ft level at the peak, at 2 ft out there is about 1/2 inch space between the ends of the level and the floor. At 3 and 4 ft out, the space is probably greater.

What's the best way to handle this? Should I use a leveling compound? Can I plane or sand down the plywood around the peak? Do I need to remove the plywood subfloor and plane down the joist (assuming that's the main cause?) I have a tight budget so planing/sanding down the plywood around the peak would be great, assuming that works. I have a belt sander, or I could rent a planer.

Suggestions?

Any suggestions on how to level out the peak in my floor? Basically, there's a "tented peak" where the two pieces of plywood are butted together.

Once I deal with that (assuming I need to), I plan to lay down 5/8 exterior grade plywood followed by a sheet of Ditra membrane (I have Durock, too, but the Ditra will be easier).

Thanks for any pointers.