Basically, anywhere you'd take mastery, take haste instead. For example, all of your mastery gems can be replaced with equivalent haste gems (except those parry/mastery gems, replace those with expertise/haste and drop expertise elsewhere to compensate). And yes, you CAN use pure haste gems if you don't care about matching socket bonuses, since a lot of them are terrible (dodge, parry, crit).
Additionally, if you can, start picking up Ret items to replace that avoidance gear. Things like the haste/mastery cloak and necklace or the Brutal Talisman from Valor are good if you have the extra Valor.
With a Haste build, you just put Haste rating above all else (with the obvious caveat that you do need to keep that 7.5% hit and 15% expertise). Mastery is still quite solid, and Dodge/Parry/Strength aren't bad at all, but Haste offers so much in so many ways. Prioritize it with gems, reforge into it, and look for it on gear if possible. It isn't a huge deal to have Haste on your gear itself, and it's inevitable that you're going to have a lot of traditional "tank" gear (especially if you're in a progression guild with plate DPS who may not have alternatives to that haste gear - I am almost always in the habit of passing haste gear to the DPS in my guild).
The thing up for debate is the relative value of Haste and Stamina. It's obvious that Stamina is always ALWAYS a good, useful stat. I don't understand gemming it over Haste rating (meaning proritizing it) unless you truly need that health to survive burst damage mechanics, and that's if you're already using dual Stamina trinkets which are much more effective ways to boost your max health. I'm tanking 25H content and I'm not seeking out tons of Stam - I don't gem it except to hit socket bonuses, and I only have to sub in one Stam trinket nowadays (I had to sub in two of them for specific fights earlier in the tier, but by now my gear has improved to the point where I don't deem that necessary or wise). 10 man bosses deal much less burst damage (basically less tank damage) because 10 mans often have to two-heal fights that you can afford to six-heal on 25, and it leads to 10m tanks putting a bit of a lower focus on Stam.
So with gems, I would really focus on Haste with them. Pure haste in yellows/prismatics, exp/haste in reds, and haste/stam (or hit if you're somehow not drowning in hit ratings) in blues. You can skip really awful socket bonuses if you want, although since you appear to be reforging into lots of Expertise to cap, there's no downside to using an orange rather than a yellow.
Two other little things that may not be directly related to your stats but are about your toon: I'm not sure why you're glyphing Consecrate but I would really actively avoid doing so unless it's truly necessary, since having to target the thing does bite into your rotation and can slow down your Holy Power generation. Also, Colossus is a pretty terrible enchant since the 8k shields don't scale with anything - it's a drop in the bucket now. I'd suggest either Dancing Steel or Windsong, I haven't seen conclusive math to prove one is superior to the other but Dancing Steel is my choice.