I've created symmetrical drawings using the "Transform" effect under Effects > Distort & Transform > Transform. After I'm done with them, I'd like to change the whole thing into actual paths. (not shapes, so expand, expand appearance or flatten transparency doesn't work for me, as far as i know) How can I do this?
I suggest an alternate workflow-
It seems you have made your original shapes with a brush other than the "Basic" brush. Any brush other than the "Basic" will turn your paths into filled shapes when you do Expand Appearance (as @Scott mentioned in his answer to your previous question).
If change your brush strokes to the "basic" brush before you do Expand Appearance you will lose the look (appearance) of your brushstrokes. You will have to change the stroke weight and the corners and caps to look like your original Brush.
The Expand Appearance is required to turn your Effect> Distort and Transform into an actual path and not an effect.
Do away with the Transform Effect (the reflection) and simply use the Reflect Tool (O) to reflect and copy your brush strokes. Alt/Option click a point on your original path and a dailog will come up allowing you to choose the Vertical Reflect option. Choose Copy rather than OK at the bottom. Then you can join your paths (as @Billy Kerr says in his answer to this question) and not have to expand and lose the Appearances you have.
If you do this again then create your paths with either the Pencil tool (as you have said) or the Pen tool. Then do your Transform Effect, then Expand Appearance and join your paths, then apply your desired brushstroke.
Select the path, and do Object > Expand Appearance. This will render the Transform effect as an actual path.
To get the corners to work as a mitre join, you'll need to join the two paths by selecting both paths and do Object > Path > Join.
You may also have to move the paths so that the two end anchors intersect, so that they are exactly in the same position before you attempt the join. Use Smart Guides to do this.