I'm a founder of a hardware startup. We're a bit further along in terms of funding but we've been where you are so I think I can offer some good insight.
probably waited too long to start trying to raise Series A.
It hurts to be too early and it hurts to be too late. Nobody knows how quickly it will happen either (if it happens at all). There're no easy answers here. The only thing I can say is that management probably has more information about the company's financial position and hopefully more entrepreneurial experience. All things being equal I would defer to their judgement here.
I get the impression our initial investor meetings for Series A have not gone very well.
Welcome to the world of hardware startups. Series A is a different animal for hardware. The dev cycle is longer and more expensive. A lot of hardware startups don't have significant revenue or market traction when they raise their A round. Unfortunately, as it turns out, that's the language VC's speak. As with any language barrier you're in for some awkward hand waving to bridge that gap.
I try and be realistic with them (I have a lot of experience in this field and a lot of experience estimating projects like this) but they don't seem to listen.
I've seen both sides of this coin. Founders are optimistic by nature and engineers are pessimistic by training. Right from the outset you have a dichotomy of viewpoints. If I ask an engineer for a timeline on a super important part of our only product that determines the success or failure of the entire company, I'm going to get a long ass timeline. Every time. If I ask a founder when their company with no revenue and no traction is going to be profitable, I'm going to hear about this coming Christmas season. Every time. Maybe I'm being a bit hyperbolic, but you catch my drift.
To see this issue from management's perspective you should consider this:
The timeline's given to VC's are contingent upon raising capital. That usually means more staff, which usually means shorter timelines. This might seem obvious but in my experience, unless you've been working directly on the planning side of things this doesn't get accounted for.
After a big raise like your A round you can start hiring specialists instead of generalists. This can turn a 12-month timeline into a 1-week timeline. That's not hyperbole, it actually happened to me.
If you think raising capital was hard before, just wait until you start telling VC's that your product is twice as far out. I haven't quite figured out the coefficients and exponents yet, but I'm pretty sure the speed of the door closing is governed by an exponential expression proportionate to the time to market of your product.
My concern over leaving is I'm the only one at the company that works in the area I am in.
"Hey $CEO, it's been a slice but I want to move on with my career. I don't want to leave you guys high and dry so let's find a replacement and make the transition as smooth as possible."
One final thought: When I was the one working for someone else I always saw things as black and white. "This was stupid," "that was wrong," "I would've done this instead," etc. I said it all. After my first few years running a company I realized how perspective changes everything. I didn't have all the information. I didn't know the real business goals. When you're working on one problem for one area of a project, you're only looking at the company through a keyhole. When you're in management, you're on the other side of the door. That doesn't mean you're wrong, but it's something to think about.