This is a very nice tree, I think that I'd want to take it to a reputable bonsai specialist in your area.
I agree with Judy
Soil looks heavy and a bit organic.
If you pinch candles back this Spring when elongation is complete but not opened much
do not pinch candles back in the weak areas. This will channel more energy to the weaker branches.
For fungal application you’ll need to spray when the candles are not fully mature and not opened
and repeat as they are opening, as well as an application of Bonide fungal treatment granules
if sold in you’re area to treat systemically, around the time of the 1st spray. Repeat annually.
The lack of light over Winter will not cause this. Needs full Sun during growing season however.
Very nice tree, good luck.
Judging by the pics, the window to repot is coming to an end soon.