Sounds like a net salary of 5k for an engineer is the most I can be hoping for.
That would be high for a non-managing engineer.
my wife may have a better chance of earning more as a physician, although she would need to specialise first.
Not in Poland. Physicians are poorly paid unless they own their own practice. That requires substantial capital to invest, and might not pay off for several years. She could, as I mentioned before, work in Germany, where she would receive higher pay. But without a good knowledge of the language and a specialty, her options and pay would be limited.
Forget about the agencies and find your own work. There's plenty out there for both you and your wife, and you can charge 60 PLN or more per page. Low startup costs and ease of finding work make this the most realistic option for both of you in Poland, especially since you both have specialist degrees. You won't have much competition. (I'm assuming, of course, that both you and your wife have near-native proficiency in English). You're going to hear the words "spadł z nieba" A LOT. Both of you can earn a lot more than as an engineer or physician in Poland. You just have to be persistent with your marketing, and carpet bomb every potential client by email and by knocking on doors until you build up a clientele, which shouldn't take long for either of you. However, you're going to have to be a rather disciplined self-starter and go-getter with plenty of business wits to make a go of it.
we already have substantial loans to pay off for my wife's MD degree
That's going to be tough to do on Polish wages. Perhaps with the translating and/or working in Germany option.
It is just that the lifestyle and emotional preference would be on PL
The States is a huge and diverse country. If you're not satisfied with the lifestyle where you currently are, check out other parts of the country. I'm guessing, though, that either you or your wife wants to be near family in Poland. That's a tough order to fill, especially in the short term.
And the Gulf - does it really still pay what it used to these days?
For medical professionals, yes. Don't really know about engineering. Warrants further research on your part.
the hundreds of thousands of Poles who have made the decision to live abroad
There were a lot of physicians and engineers among them. You're going to have to think outside the box to make a go of it in Poland, especially with debts to pay off in the States.
Like I said, your situation isn't completely hopeless. It's just that you have defined your dream so narrowly that it's affecting your perception of your options, which are manifold in the long run, and not bad for the time being. Also, like I said before, neither of you is getting any younger, and self-improvement is going to be harder, and pay off less, the longer you delay it. A bit of creative thinking together with draconian self-sacrifice and financial management now can put you both in a truly enviable position five years from now. At your age, self-improvement should be your HIGHEST priority. To the point of selling the house and buying a smaller one, if necessary.
Another priority is getting your kids the best education they can get, and, frankly, in spite of your experience, a degree from an American university opens up a lot more doors than one from a Polish university. Make sure they are working their asses off in school right now so they can get into the best universities possible, and get scholarships. They should eat, breath and dream math and sciences. If your kids are slackers, probably the worst place for them is a Polish university, where nobody is going to care at all about them. Also, if you kids are slackers, get out the belt and unslack their sweet little behinds before the sun goes down. They will be eternally grateful.