I feel you!
In some circumstances we are having a hard time bursting adds and it depends highly on fight mechanics.

Generally when I learn the fight, understand how my group handles particular phases, I start anticipating phases in order to provide as much dps as possible.
By anticipating I mean timing my Aetherstacks and CDs, as well as dots on main target or possible target for bane.

From what you say, you do quite fine. What I would add to rotation is swift shadowflare if mobs are not expected to move away. People underestimate this skill, while it hits way harder than miasma II - dont remember exact number as I am at work, but i.e. miasma II will hit on average 30dmg per tick, while shadowflare will do 80-90dmg per tick.

If adds spawn close to my main target, which for majority of the time will be boss, then I always try to time my dots to be fresh on boss, keep aether stacks up, as well as contagion. This is quite easy scenario as you just have to bane them, but seen many summoners not thinking ahead with their CDs and Aether on such cases.
Also, if adds are expected to stay in one place and not spread all around the place, then you should keep swift off CD and cast on spawn shadowflare in their spot.

In case you cannot bane and have to dot everything from the beginning, then I always try to ask myself a question on how long those adds will stay up. if the HP is low and burn is high, then just do what you said(miasma II, blizz, ruda's aoe) with addition of swift+shadowflare (in case they are not expected to run away).
In case of DPS check which lasts longer and adds are spread, dot as many as possible and try to use contagion on the add which is not the priority target for rest of your group, best 2nd or 3rd so you get the most of your dots.


Also, it is a huge help if your teammembers have knowledge of other classes than their own and know how to improve overall performance of the group.
Seen plenty summoners who i.e. burn their festers asap on Twintania and are on 30 sec CD for any burst, while in few seconds comes conflag so they cannot bane+fester and the conflag gets long.
I think the key for proper burst with Summoner is anticipation of upcomming mechanic and taking it into account before you burn your all Aether stacks and/or CDs on boss, who usually does not even have a need for bursting other than standard dmg output from the party. In fact, sometimes it is even recommended to slow down due to fight mechanics.

Regards,
M