/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.
The challenge is, as soon as you start systematically introducing psyops into a quality discussion, every message becomes suspect and the quality retreats. What would happen if every thread on /r/askhistorians has someone popping in and saying "I see you're talking about the Battle of Gettysburg! Did you know there's an etsy shop you can buy official artifacts over [here]?" And more to the point, how will /r/historians defend against professionals steering discussions about, eg, slavery or the annexation of Tibet or the religious views of the Signers? I am worried.