choppy slow motion

This looks like the clip was recorded with a high shutter speed.

What you see is probably just a combination of that and the relatively low 23.98 frame rate. (*)

You might try adding a little blur.

One common technique is the following: duplicate the clip and place the two copies one above the other. (Option-dragging is an easy way to do it)

Nudge the top clip so it is one frame off. Set the opacity of both clips to 50%.

This will add a bit of motion blur and make the movement look a bit smoother.

(*) <rant>oh why don't these fractional frame rates die, already!</rant>

I agree that we would not expect this.

I think the little trick I mentioned before gives quite a decent result.

I have made a short before and after video to illustrate:

https://youtu.be/U4pktYkO4y0

We would have to see the actual clip to see exactly what's causing the problem. The shutter speed might be correct for 59.94 media, but when you set the retiming to auto you're effectively giving the high shutter speed to 23.98 media. You're better off using a higher frame rate than simply doubling the project frame rate.

Movement in video shot at 24fps has motion blur on the frames. Because of your high frame and shutter speed there is no motion blur on each frame. When you expand your video so each frame of 60fps is on a frame of 24fps your frames do not have the blur you would expect to create the illusion of smooth motion at the low frame rate. Your video appears to skip between frames.

Would it be possible to post one short sample clip with the same settings so we can test?

You could send it to yourself using wetransfer, then post the link here.