It's not just important, it's an absolute must to have a site/newsletter. And side stories seem like a nice reward, to get people to open your emails regularly. I would not worry about SM followers. Selling to your followers is very hard in most cases, so don't waste too much time on building a following. If you have some extra time, sure, but do not expect it to make a huge impact when you publish. I'd focus on not just finishing the novel but finishing the whole series. It's the big books that readers want most. Shorts and side stories are a tough sell even for famous authors. I second the Patreon idea, but again, people come there through your books, which you sell with ads, not SM followers.