I believe we're not supposed to overthink it. What do musical legends have in common? Cultural impact, global fame, record selling and chart-topping albums, and shaping future generations of musicians. Most of these can be achieved by just making music with universal appeal and getting really, really famous.
I would love to just enjoy the art, and I do, but everytime the legend is mentioned, I can’t help but to be reminded that being a “legend” is literally the driving plot behind the story and a big part behind Shiori’s motivation of doing what she does (cause she doesn’t want to die by her next birthday), including this chapter. And yet it’s vague that it could mean many things.
Being impactful as you mentioned is a valid point that I lean towards agreeing, but in a modern setting this also requires different means than during the times of the ghost musicians (among other complications).
Beyond that, it requires time, which Shiori doesn’t have. I would argue that the legends cemented their status after their death, which means it took them their (relatively short) career length + time after they passed away to truly become legends. Shiori has much less than even their career length…
This is a long way of saying the art is amazing, but the vagueness of the plot bothers me from truly enjoying this great series because I can’t help but to worry that something stupidly anti-climactic is waiting for us when it ends.