Thank you so much for your detailed comment!

Basically, I completely agree with you:

In general, symmetry is not required. In my case, I wanted to include it in the picture, which is an additional factor to practice precision.

I only use the OM system and its live composite mode in workshops or for practice. I'm just too lazy to run to the camera after every picture and check where I've put my light trail. Of course, I don't have my eyes on the screen when I'm drawing a light trail.
It's only worth activating the Canon once I've memorized the coordination in my head. I think I've done the practice round now and I can leave out the OM system next time.

I'm right there with you, it's unsporting to use a live composite mode when lightpainting. The image should at least always be labeled when live composite mode is used.