You need to re-read the rule book. Either it or your memory has changed.
The rules require the outside of the rack to be marked.
Among other issues with marking the inside, there are positions for the break ball near the rack that would be unplayable by that method.
The Dritz fabric pencil wears off in a few weeks. There are other kinds of chalk pencils that are thicker. I used to have a mechanical one that had 2mm chalk refills, but they tended to break a lot. There are also wood pencils with about 2mm chalk leads.
I think thinner is better because it is less conspicuous. It is still visible when you need to see it.