Very cool project. Was wondering how you managed to get the pieces to fit in the slots as I too had problems with accuracy when cutting a drawer with interlocking tabs. In my project each edge was 1/32" (0.03125") off which meant the interlock was 4 x off, as mine had both interlocking pieces being cut, and was 1/8" (0.125") too big overall.
I found settings in Fusion 360 -> 2D Contour -> Passes Tab -> Stock to Leave -> Radial & Axial Stock which might fix this problem.
These can be set to a negative value which will remove extra material from the inside of the selected path. I have not tried it yet so do not know if this work.
In your case you only have the slot cut in one piece and the other piece is fixed width. If I understand correctly your original CAD file had 0.19" slots that ended up being 0.16" when cut, which is error of 0.03", which you compensated for by making the slots in CAD at 0.21". So you could try a negative stock to leave of -0.03 / 2 = -0.015", For BOTH Axial and Radial, with your original CAD file.
This will remove 0.015" from the entire path, making your overall project that much smaller, but it should be a much easier fix then adjusting all the slots in CAD manually and will not be noticeable visually.