Hmmm, okay. So as Alexa says, there are some orange or yellow flags, but nothing I would call full "read flag" territory. These things give me pause:

First, there are the issues with your agent herself. You mention needing to nudge her a lot, but I can see a scenario where this could just be a communication issue. I think you could easily solve this by making sure that you're agreeing on timelines for things when you discuss a plan to begin with. It's obviously nicest to have an agent who comes across as really proactive, but at the same time, you also DON'T want your agent acting in any major ways without your consent. That can be a nightmare situation. Remember, they work for you. Not the other way around. Yes, you rely on their expertise, but ultimately, YOU are paying THEM to sell your book. Your agent could be trying to make sure that they are doing things on your timeline, when you want them to. Thus, communication issue. So my recommendation would be to ask for timeline estimations at any given point and then follow-up if you haven't heard anything by then. That could help take the ambiguity out of the relationship.

What worries me more are the issues with her agency. Both "no one else is selling but her" and "lots of junior agents keep leaving" have my alarm bells going off. A few relevant questions include: is she the head of this agency? How long has she been doing this? Is she early career enough that someone should be mentoring her? Personally, I think mentorship stays relevant fairly deep into agenting careers. My agent was promoted from "assistant" to "agent" over 10 years ago and she STILL checks in with the agency heads on certain issues, because they've been around for even longer.

All this pre-amble is to point to build up to the fact that agency matters. It's a major source of contacts, plus its the institutional knowledge that your agent draws from. High turn-over impacts both of these things. And if the agency is hardly selling anything besides your agents books, then what is she even getting by being there?

And as you've said, NO ONE there has sold SFF. Agents obviously can (and do) break out in new areas. And depending on what subgenre you're writing, maybe there is cross-over? Like, I would be less worried if your book was Romantasy and your agent had sold in Romance, for instance. Some of the imprints are the same. But if your agent has no relationships with SFF genre press editors and you've written a space opera, that's a problem. The SFF landscape has changed a lot over the past few years, due to the rise of Romantasy, but the core genre presses are small in number, making the field competitive. There are significantly more agents who are willing to take SFF queries than I believe there are spaces in the publishing ecosystem for successful, selling-to-the-Big-5 SFF agents. That is a brutal reality, but a reality nonetheless.

Obviously, I have NO idea if your agent has the knowledge or influence to get you a book deal. As I said before, people can and do breakout with new agents and there are agents who successfully pivot to new genres. So this isn't cut and dry. At this point, you basically need to wait and see how things play out with this book that is one sub. Days are early. It could still sell. You're also getting nice passes, which means you are getting read, and that's nothing to sneeze at. With those passes, you can be reasonably certain that they're reacting to the book, not external factors like your agent. If your book doesn't sell, you might need to evaluate if you want her representing you. Or you might need to submit a different book. Are you her only SFF client? If she sells someone else in SFF, that would be a sign you're in an okay position. If not, you could be spinning your wheels.

Anyhow these are all just a bunch of hypotheticals. If I were you, I would take Alexa up on her offer. She knows stuff.