There was a forum post last month for the demo! The author also has a Patreon, and he announced book 6 on there in October (I assume it took some months for editing, submitting to HG, etc.)
It’s easy to miss though if you’re not regularly checking, especially without the usual upcoming HG pinned post on the forum, it does feel like they release out of nowhere. I’m not sure why there still isn’t one even though we’re almost halfway through the year?