Man, I was in the same boat after that ending! As far as I can tell from official sources and all my digging, there isn't a direct sequel novel series that continues right after the events of the main story. The web novel and light novel adaptations for the core 'To Be a Power in the Shadows!' story are, for now, complete. The author, Daisuke Aizawa, wrapped up Shadow Garden's big confrontation with the Cult of Diabolos in a pretty definitive way.
That said, the story definitely continues in spirit. There's an ongoing manga adaptation that's still serializing and hasn't caught up to the novels' ending yet, so you can follow that for a different pace and art style. Also, I hear there's a series of short stories and side content sometimes bundled with special editions, focusing on side characters like the Seven Shadows. It's not 'Volume 5,' but it gives you more of that world. The anime's second season also covered a good chunk of the later arcs, so if you're just looking for more animated content, that's there. I'm holding out hope the author might revisit it one day for an epilogue series or something, but for now, what we have is a finished, if slightly abrupt, package.