Is it possible to have the adverb at the end in this sentence (and generally in sentences including adverbs of frequency)?
It might be possible, but it’s much more natural to put it in the beginning.
Why are both artystką and artystą interchangeable? Is it male/feminine?
Yes, or rather the other way round: "arystką" (artystka) is feminine, "artystą (artysta) is masculine.
I know this has been discussed before, but I am not understanding it. Why is artist instrumental and not genitive (because of the negative)?
See this guide: https://forum.duolingo.com/comment/28545847
tl;dr: Only Accusative changes its case when negated.
why it is not : nie jestem artystka jeszcze ..
its always confusing ;-)
It doesn't really sound good. It sounds mostly better if you put 'jeszcze' at the beginning of a sentence, but I'm not sure, as I'm not Polish...
Why can't I use Juz (dot over the z) rather than Jeszcze?
Because "już" here would mean that I'm not an artist anymore.