I can't help with a route but have some experience of Polish roads and traffic. They vary in surface from super smooth to deep sand - sometimes the most direct route, as well as the most scenic, is through on forest roads - but tend to be more rutted than potholed. You'll find some cobbles in town centres too. The traffic tends to be ludicrously fast but IME respectful and careful of cyclists, giving you a wide berth. They prefer to terrorise pedestrians and each other... Outside of bigger towns, there isn't that much traffic, though you'll be in Silesia (Śląsk) which is one of the most densely populated and industrialised regions.
I don't think campsites are too common and wild camping is, I think, illegal due to fire risk, but in practice I know people who have wild camped without problem. Hotels run the whole gamut.
Thinking about the route now, going through the Sudety mountains might be a nice route - depending on how you like hills!
