Many questions are one of
and some other issues
I feel the mods should filter the questions too, for the same reason we filter answers, for quality.
There are some other history forums on the internet, which follow this approach(though none as popular as AskHistorians). One of them is history stack exchange, who's policy i find interesting
This might also mitigate our issue of a lot of unanswered questions.
There are "standards" (more like restrictions) for the questions that are asked here. You can find them in our rules for asking questions. We also have featured posts from our Rules Roundtable series where we explain how "There Is No Such Thing as a Stupid Question" and offer a guide on "How to Ask Better Questions" so users know not only what kinds of questions will yield the information they're looking for, but also how to generally improve the odds of the question being answered at all.
Rules Roundtable #16 actually addresses some of the concerns you've raised. Regarding questions that might appear "too opinion based," we note:
Often times a question will be asked in which the underlying premise of the question is off. As long as it isn't in a manner which violates the rules concerning Soapboxing in questions, we will rarely remove a question simply because it is based on a faulty question. We try to be understanding and accommodating that the very reason someone has come to the subreddit is to learn, and removing the question generally won't help the user who asked it, but rather would punish the user for their lack of knowledge. Our decided preference and hope is that their misunderstanding can be corrected by users or flairs of the subreddit, and so those questions are allowed to remain.
And regarding questions that might appear "too broad," "too specific," or "too obscure," we note:
As exemplified by the "Example Seeking Rule", we try to ensure that questions don't end up too broad in scope. But the flip-side, of course, is the need to balance those constraints with the simple fact that, like with falsely premised questions, laymen coming here to ask questions might not know enough to make their question as specific as some might like to see.
Each moderator has an initial discretion for evaluating the questions as they get posted to the sub and makes a judgment based on their understanding of our rules. If one ever believes there is an error, the OP can appeal it through modmail or other users can report something they think breaks the rules, to which a moderator will reevaluate the question (and we do check every single report that comes in, which is currently "0" at the time of writing this!).