I think you should redraw it, because it would take longer to fix than it would to redraw.
When you draw your line, don’t worry about the curves, just make straight lines with normal corners and apply a thick black stroke.
Now use the Selection Tool V to select the entire line.
Hold down CTRL and you will see the Live Corner circles appear.
Click and drag on these to round the corners
On the corners where you don’t want rounding, click the Live Corner circle for that corner, then click and drag it out to make it a regular corner.
You can also use the same trick on the big black rectangle.