I've noticed that the good subs tend to feature least two of the following traits (or at least minimum overall parameters): active moderation with a clear focus, a relatively low population, and topics that are inherently less motivating to psyops in the first place.

/r/askhistorian's population is fairly high in absolute terms, so they make up for it with scorched earth tactics and a specific culture that has evolved over the years. And while it's true that history is juicily controversial, marketers and trolls have opportunity costs to take into account, so they largely stay away from such a tight environment.

The problem you mentioned about steering bias is not in fact solvable. By definition, just having the site be in English and its audience be anglophone already dooms it to hopeless levels of bias. But the fact that the sub is attractive to a wide range of actual historians providing quality content means that they've more or less achieved what could be realistically achieved. 21st century academic culture in the anglosphere is what it is and we can't escape that. We can however cultivate the best version of what it can bring to a lay audience.

If this compromise is not tolerable, the solution is in fact to stop using the platform altogether.