Yea that is completely just, bleh. The opening rotation is something like

Obey | RS -> Bio II -> Miasma -> Bio | Fester -> Thunder -> Miasma II | Contagion | Swiftcast -> Shadowflare |(whatever you want here, meh, possibly energy drain if you want to aetherflow earlier) -> Ruin II | Fester-> Ruin II | Rouse -> Ruin II | Spur -> Ruin II | Enkindle.

I'm pretty sure Kevee has something slightly different, but I'm too lazy to look it up. Furthermore you do not have to enkindle, generally save enkindle for an AoE situation, but in a single target boss fight w/ no adds sure why not.

The idea that "dots tick every 3 seconds" is sound, but not your application of it, for example if you miasma after Bio, there is a small chance that the internal tick will pick up both for the first tick.

Anyway no GCD ability is ever free...the cost is always a GCD, its just whether the damage is frontloaded or backloaded.