Not sure why it would warp there, perhaps infill percentage is too high?

If you don’t need the layer lines to be in a specific orientation I’d consider rotating the model back 90° for the reprint, hopefully that should avoid any warping.

Why not both? (Insert old meme here)

I want the Baltic Sea surrounded in a similar fasion as to how The Roman Empire once enveloped the Mediterranean Sea!

i.e. A United Nordics + Balticum + Poland + Germany (maybe just the parts of Poland and Germany that are near the sea though?)