The channeler, as you've described it, is most closely modeled in Pathfinder with the alchemist's bomb ability. Targeted elemental energy with class choices to modify it describes both abilities, so I'd look to the alchemist for how to set that ability up, and alter it based on the fact that you want it to be completely at-will.
What you're describing is also (obviously) pretty close to the warlock's eldritch blast, since that seems like a good chunk of the inspiration you're going for, with an elementalist vibe. That makes sense, people seemed to like the warlock as a ranged combatant with magical tricks.
I'm not sure how much I like the idea of being limited to one energy type up until 5th level; I believe the alchemist can change their bomb's damage away from fire pretty early on. Warlocks never had to worry about elemental resistances and such, so hampering other elements significantly seems limiting.
Increasing the damage die feels like a strange mechanic to me, but it does avoid having to roll a huge pile of dice to keep the damage about right. 11d12 at level 20 is pretty nasty for a caster, but a fighter of that level can do some pretty sick things in a full attack. I don't play at those levels, so I'm not sure how well this fits in DPS-wise with what's expected up at the higher levels.
The overcharging mechanic could just be a concentration check, with DCs based on how much they want to overcharge. Each action they concentrate on building up the charge, they make a concentration roll, say DC 10 + 2 or 3 per extra die. If they fail this roll they take some damage (all of it, maybe?). You could bump up the DC if you wanted to build up the power faster, too. I'm not sure how often it would be used if it had to build up like that, though, since it would usually be a terrible waste of actions to do that in combat, and walking around with a glowing ball of energy you were concentrating on wouldn't work very well.