You should be able to add your own sticker sets in the Web version like you can in the Apple version. There are so few stickers built in, and many Goodnotes (planner) purchases that I have made include stickers that I love and use often on my iPad and am no longer able to use on the browser.
I've seen this suggestion before, but it never specified adding to the web version. If this is a functionality that is already built into the Apple, why can't we add it to the browser?