Hi Atch,
yes we actually plan to do that, even though it may later complicate the legalization of the marriage in Poland (again translating all of mine and her documents into Polish and verifying with Polish authorities, which may take a long time as I heard). And we would probably need that in order to get her a Polish temporary residence permit based on that marriage (or not? would it just be enough to translate the marriage certificate by a sworn translator and that's it? don't know..)
But as I was reading further, I am pretty sure she wouldn't need any visa after that, she would just apply for a temporary residence permit/card based on being married to me and that would allow her to stay for as long as that permit would be valid (and travel around Schengen, and later apply for a new temporary residence permit/card).
And she also wouldn't need to travel back to her home country.
I have found out that a 3rd country person can apply for a temporary residence permit/card even based on an informal relationship (but yes, needs to prove the relationship with sufficient evidence and regular/stable income - and that can't be done by the informal partner, unless they sign some annuity contract, which I have no idea what it is) [attached screenshot]
So it looks pretty positive. Now we just hope she actually gets the visa.
these are some useful links in case someone comes across similar issues
migrant.poznan.uw.gov.pl/en/procedury/i-am-informal-relationship
infoopt.pl/pierwsze-kroki-podstawowe-informacje/
infoopt.pl/legalizacja-pobytu-obywatele-panstw-trzecich/