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Schizo here.
I've spent at least 100h browsing google maps and maps on population density, average temperature, intact forest landscape, etc ... and I've come to the conclusion that your best bet in Europe is the Carpathian mountains. Either the part that is in Ukraine or the part at the Romanian-Serbian border (pic related). Another option would be the Southwestern Caucasus foothills. Very forested, quite the warm average temperature despite the elevation, barely anyone living there and mostly old growth forest. Another plus is that a lot of chestnut trees grow there which is an ideal food to survive the winter on.