A marketplace like bandcamp would be more interesting IMHO, I regularly buy digital artworks from games or movies, that’s more analogous to buying an album on bandcamp.
The marketplace you propose could be interesting, but IG already existed with a huge number of artists who’ve already had a following/status/lore. Also there is the exposure to a tremendously broader&bigger audience than any other platform could possibly have. Not saying that migration would be unachievable, but so much would be lost for something that I don’t see come close to being a compensation
What you seem to want is a kind of tipping or patronage system integrated with Instagram if I understand correctly? Sounds like a cool way to support artists. NFTs are a very contrived way of achieving this.