You keep using Garuda feathers as an example, what about just clearing trash in dungeons such as AK? Turn 1 also has aoe trash before Cadaceus, Turn 3 and 4 are full of AoE packs. Are you suggesting its more efficient to just spam flare and transpose on the packs?
As for Blizzard 3, if you have Astral Fire 3 up its a 1.66s cast. I believe the goal of the rotations being recommended to people is to manage your mp. You fire 3 > fire spam(using Firestarter procs as they come) until you hit near a certain mp point, which for me is 504(If I'm using T3) or 398(T2). Once you hit near the magic number you slam in the Blizzard 3(79 mp with AF3 up, 1.66s cast) while still under the effects of AF3. This immediately pulls you to UI3, where you refresh your Thunder. Once the thunder is refreshed, you cast Fire 3(casting times halved to 1.66s again thanks to UI3) and begin the cycle again. My MP refills completely right at the Fire 3 cast under UI3; If I used transpose at the end of my fire rotation instead of Blizzard 3, I'd have to refill with just one stack of Umbral Ice. In the time it takes to regain full mp with just UI1, I can fire off 3 Thunder 2's to give you an idea of the time spent waiting.. I suppose you could argue that you just go back into fire at half MP, but that seems like a waste, imo at least.
As for bosses and movement, I understand your concern but as you said just about all of them require some form of movement. My main is a SMN and it takes me roughly 7.5 seconds to get the Bio2>Miasma>Bio1 train going. Add in another 2.5 for Thunder and you're talking 10 seconds. Add in another 2.86s cast for Shadowflare. You see where I'm going here, It's just something that you deal with on fights like Titan and up really.