Marisa's intentions are vague enough to support either interpretation. Will she end up remaining an ordinary magician until she dies of human lifespan/frailty, or find a method of immortality she likes enough to commit to?
And if that means she does become a youkai, is there a statute of limitations on being born a human in the human village such that a decade or two spent as a crazy forest witch first would make it acceptable? It's rather nebulous and open to speculation.
I find this work interesting because of the inclusion of Marisa's usually absent mother. It seems to have her find out that Reimu specifically ended her late enough in life that they were already friends and she already understood how things work. So instead of any long term efforts to avenge that, Marisa takes a 'that's just how things are' mentality. If she continues on her path long enough, she and her best friend will have to fight to the death over it and that's just life. As far as angst goes, it comes across as a very dry sort. She's not dreading that day nor trying to make the most of the time before it, merely acknowledging the inevitable before progressing as she otherwise would.