The studies show that second generation of migrant (from Asia and Africa) women don't have that many more children than Europe's natives (contrary to what the populist anti-migrant propaganda is pushing) - in most countries the difference was on the order of 0.3 - 0.5 children per woman if I remember correctly.

Poland tries to address the fertility issue in a different way - we're letting in lots of Ukrainian migrants (currently there's 500k of them working legally in Poland, with probably another 500k working illegally). They are a much better match, as their language, culture, values and even parts of history are close to ours.