@Toxxi, pretty good advice

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A 0,03 from me

Poland is now changing. We have a few regions, who started to develop the "European" style cycling tourism. They build new tourist cycling paths - mostly along the rivers and in the tracks of former railway lines.

Three of them are the leaders - Western Pomerania (Pomorze Zachodnie - Szczecin), Lesser Poland (Małopolska - Kraków) and Pomorskie (Pomerania - Gdańsk).

And there are also a few smaller "projects", a few short routes, which you can describe as "not bad".

All of them, I think, you can find on my blog.

Poland by bike. 10 best bicycle routes [2023]:
https://www.znajkraj.pl/rowerem-po-polsce-10-najlepszych-szlakow-rowerowych

It's only in Polish, so you need to use translator. You will find gpx files there (ours, not the official ones).

If you think about 10 days route, go to EuroVelo 10 and in Elbląg change to Elbląg Canal Route and finish your tour in Olsztyn.

You can also think about the Green Velo route. You won't find cycling infrastructure there, but the attraction of it will be the atmosphere of Eastern Poland. We call it "Exotic Poland". So it depends what your cycling needs are.

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About trains:

There are two quite stable train connections: Berlin-Poznań-Warszawa and Berlin-Poznań-Gdańsk-Gdynia. In the last ~10 years they were changed only when DB was renovating railroute between Frankfurt/O. and Berlin

All these trains have bicycle compartments - a good idea is to buy a bicycle ticket in advance, because 8 places is sometimes too less.

Trains on Berlin-Wrocław-Kraków/Katowice/Przemyśl sometimes change because of various reasons.

Yes - regional trains to the PL-DE border are also a good idea. Except the cities mentioned by you, there are also Forst, Kostrzyn and Szczecin.

Any more questions - go on

Szy.