What I didn't like ...

All in all, I think it went fairly well. But there are one or two details I would change if I were invited to do the performance elsewhere. (Like all performance, doing it again, actually means doing a new version - an art performance can only be done once - which it distinguishes it from a theatrical performance which can be repeated.)

I would not use the architect's plan-paper. Although it was possible to see the act of drawing on the reverse side (visible as intended to the passing external audience), the material itself was too rigid, and the ink only remained on one side.  I need to find a source of very large newsprint so that the ink could 'bleed' and literally be present on both sides of the paper.

I felt the plan-paper I used didn't show the markers to their full potential. This is particularly evident in the video where some colours can hardly be seen at all.
Another issue with the markers was that if I held them at a certain angle, the line was so thin as to be invisible at any distance. Yes, I could have practised some more before the event, but for me, the essence of my performance is that it is unrehearsed, so that neither the audience nor myself know exactly what is going to happen. So I need to accept that this quality of the line is one of those unexpected incidents that are part of the event.

The video is reasonably OK, particularly for an unattended camera. However, I really need to identify a reliable camera-person, or even more than one, whom I could call on to video when I perform. Money is the problem of course - would there really be someone who would volunteer to do this, and do it reasonably well.