FWIW I am tapering off Lamictal this week (new p-doc's advice, not my decision), and it goes like this:

from 100mg at night to

50 mg at night for one week

then off completely

Obviously since you are taking twice as much for starters, you will take longer to taper off.

I was taken off a huge (more than twice yours) dose of Seroquel XR in favor of Lithium, at my request, in a matter of several days. But it was on a psych unit - I guess, they can afford to move things along more swiftly. From what I read on this board, tapering off Seroquel at home takes time.

I have never taken Zoloft.

Of course, your doctor will be the better source of advice. But I do recommend ranking these medications in terms of perceived unpleasant impact on you, and tapering off one by one - may be you will like being somewhere in the middle. Just do not stay on Zoloft without the protection of Seroquel - you might go manic. Just thinking out loud.