My main is a disc priest that has done all the raid content including 3 drake sarth so I thought I'd put my ideas in.
This is the spec I play as. I don't put any points into renew as someone else as said, it is sort of worthless to the mechanics of a disc priest. IDS is worthless for me as our GL is a shaman and our other resto shaman is very good about her attendance. If you don't have a shaman i would say get rid of desperate prayer and one from divine fury. Although, tbh, a casting shaman is beneficial to your raid that getting one or an extra one is a plus. Glyphs are going to be shield and flash heal of course, but for the 3rd i sometimes put holy nova. If you find yourself having to support you party when fast heals then this can be a very beneficial glyph, if not i do suggest the dispel glyph.
The flash heal VS greater heal fighthas been raging since disc has become a viable spec. My personal thoughts on it are both are great for the spec but its very situational.
Situation 1: Patchwerk
I am usually the only healer assigned to my tank in this fight so my thoughts may not be the same for you. The damage that is being taken and the pace with which it comes it high enough that greater heal is not a viable spam option for most people. This is where I would use a PWs, pennance, FH, and PoM rotation to keep him at constant manageable HPs.
Situation 2: Sartharion 3 Drake
This is where i tend to go a different way. I am always the MT healer for this with one other player(never the same person unfortunatly). This fight i take out the FH from my rotation and put GH there instead. Flash heal is used only as a quick buffer heal when weird things happen
If you enjoy playing this spec check out the changes on PTR. I have been on there with my character to get used to the changes and even though the mana regen nerf has also hit us, there is a lot of good changes comming for this spec