I think that it would be theoretically possible with a modified client. But in practice you'd filter a lot of genuinely active users out, and still let a lot of those suspicious accounts in. Sadly I think that blocking them individually is a better approach, even if a bit more laborious.
On a lighter note, this sort of user isn't a big deal here in Lemmy. It's simply more efficient to manipulate a larger userbase, like Twitter or Reddit.
I like this piece. Well-thought, and well laid out.
I do believe that mods getting weathered, as OP outlined, is part of the issue. I'm not sure on good ways to solve this, but introducing a few barriers of entry here and there might alleviate it. We just need to be sure that those barriers actually sort good newbies in and bad newbies out, instead of simply locking everyone out. Easier said than done.
Another factor is that moderator work grows faster than community size; you get more threads, each with more activity, users spend more time in your community, they're from more diverse backgrounds so more likely to disagree, forest fires spread faster so goes on. This is relevant here because communities nowadays tend to be considerably bigger than in the past; and, well, when you got more stuff to do, you tend to do things in a sloppier way.
You can recruit more mods, of course; but mod team size is also a problem, as it's harder to get everyone in the same page and enforce rules consistently. If one mod is rather lax and another is strict, you get some people getting away doing worse than someone else who got banned, and that makes the whole mod team look powertripping and picking favourites, when it isn't. (I'm not sure on how to solve this problem besides encouraging people to migrate to smaller communities, once they feel like the ones that they are in are too big.)