I think Instagram is what every artist are use to boost his/her popularity and gain followers. The artists community there are huge and the layout of Instagram make it easy to scroll and discover your art. They have daily/weekly and monthly challenges you can participate in.
You can use instagram hashtags to let other people discover your art. Like in this post you can use #DemonSlayer to let the fans of Demon Slayer discover and like your art and maybe follow you for more.