with the constant promise of "opportunities for growth" as the company
Startups work on mutual trust, and you can't stall a raise for your employees with this vague promise. They have families to take care, just like yours.
Plus, we are quite top-heavy, and each time we make an acquisition we
bring in a bunch of new directors. I see no future, but I have no
experience and I might be wrong.
Acquisitions are made when you are kicking ass (yes, literally). They are for expanding or strengthening your position in the market. The acquiring company should be very confident enough, and well off for moving ahead with the deals. Here in your case, I don't think you're ready or confident enough for that.
As a failed startup founder, let me make it clear for you:
You employees are the ones who would be driving your company on their
shoulders, and would also be the ones who'd be there for you when the
ship drowns, but that is only when you treat them well.
No one would be convinced by the promise of "growth opportunities". They're not still out-of-college kids for that. You need to literally make them feel that there is indeed an awesome growth opportunity. And make sure, it is AWESOME, as you are fighting in a very competitive world where there are huge bucks and share options everywhere.
In startups, is it common to offer just a fixed salary over several
years? Or is it a red flag?
Depends. If a startup is well-funded, then there would be hefty salaries. But, as they are rapid growth environments, I definitely see a fixed salary for several years as a red flag.