That last platform is WAY overkill. If you're building a platform to last for several years in a heavily-used olympic lifting gym MAYBE this is appropriate.

One recommendation to the person who's horse-stall mats move: construction adhesive. Comes in a tube, like caulk. My other recommendation is to lengthen the platform slightly, at the end, make the plywood full-width at that point, and bolt the rack to the floor through the platform.