@Mark P, this is very good news. I went and had a look at the spreadsheet.
All the "classic" markers are known to me, since my childhood. The "starry" markers are pretty and I could not remember thoses easily, for example to look up an end-note in an appendix of a paper-book. Why? Because I can not name them, like I could name "k" or "8" or "date of birth" or "date of death" or "paragraph" or "infinity" symbols.
I would guess that some of those could be "hard to read" when used as tiny super-script in areas with poor literacy. This is not a criticism; please do not remove any item from that list.
Our project has vowed many years ago to publish every output in 16 pt or larger, and that helps. And we are not shy in using not-tiny callers. Since ours use circles, they do not so badly disturb the normal reading, since they do not get confused with regular letters.
I am delighted, that other people have also had the idea with the enclosed or circled numbers.
I know that the encircled characters also exist as lower case and upper case latin letters, starting at U+2406. In case some project might need "many callers" for some special situation like study notes.
So for myself or for people in our audience, a letter in a (not too small rendered) circle might be easier to distinguish than, for example U+2724 versus U+2723 depending on font and size. This is, because we have all leant to look "where it matters", when we learnt to read. I can easily distinguish p, q, a, d, b because I know what matters, and I still struggle with Hebrew letters, where all is tiny and looks the same to me.
So again, you asked for feedback and you are on the right track. That will be another friendly and useful feature in PTXprint. Please greet the team and pass my thanks.