Thank you for the comments so far. Let me address what I can.
1. I am not interested in adding moderators. That is a significant time investment (and trust investment) to setting up a moderating system. Currently, rule-breaking posts are very rare, and are almost always reported by multiple users when they appear. However, as someone else put it, the majority of these conversations are staying within the official rules, but are not content that really belong on a learning system aimed at all ages. (This is also why DMs were removed on renshuu - my inability to handle potential issues in private conversations).
2. I do not think adding more rules will really fix it - rules need to be fairly clear, otherwise it becomes relatively easy to skirt by them. For example, "no spam" would be hard to enforce in the current megathreads.
At its core, though, it is a mismatch between how some users are using the "anything goes" forum and how I feel would be ideal for the majority of users. To put it simply, this is not a place for "conversations" - we have numerous channels on our discord server for that exact purpose. This is for discussions - those are longer messages between (often) larger numbers of people, with less of an immediate back and forth. I hope this is clear enough of a distinction.
If you could replace the content of a forum thread with a phone call or a voice chat, then it would be much more appropriate to use discord, or take it off of renshuu.
To be clear, my goal is not to say "don't converse" - it's to say "we have a better infrastructure for it, so please continue to do it, just use the right tools for the job."
renshuu is a 2-person company, but while my wife does amazing work with the artwork, voices, sentences, etc., I do the development, maintenance, user-support, and day-to-day. We have no desire to make the company bigger - that would be a lot of stress for us, we'd need to monetize more of renshuu (which we don't want to do), and I feel that renshuu would cease to be what so many people like about it. So we need to be smart about the decisions we make so that something like a subsection of the forums requires so much time/energy from me that the core of renshuu (the learning!) suffers.