Personally, if you're using mouse/trackpad, I'd recommend to draw your final drawing on paper, and then render it in SAI. Utilize the "Linework Layer", and the Pen tool there. You can edit the curve and thickness on each pivotal point manually after you draw with Pen tool, which makes the illusion of tablet working there.
As for the coloring... Hm, I like the shading on your current profile pic better, to be honest.
It could also be due to the darker lineart to make a 2D-ish overall look which fits your style.