My son was on Lithium for 16 months with good results, levels ranging from .8 to 1.1, but like you child, always doing better in the 1.0 range.
In June of this year, he went downhill, also.
He was on a mix of Lithium/Abilify. The Abilify started to cause too many side effects, and as we went down on that, he started deteriorating.
I will say this, we thought the Lithium was doing nothing. So, we took him off it.
The Lithium was doing something.
You don't see it until it's gone lol.
Alas, too many side effects that I, personally, could not deal with for Dylan.
I hope you work it out. The Strattera was a bad one for my son, also. Made him worse.