Law / My Karta pobytu in Poland is almost expiring - I need new visa! (I am South Korean) [2]

Hello, I am currently living in Poland for 5 years, and I have my karta pobytu as I am a student (I am South Korean).
This year my study is finishing, and my Karta pobytu expires in late June. But my friends are planning to visit Portugal in August as our last trip and I really want to join them.

I was wondering how i could extend my visa until around mid August, and I got some information that I can leave Poland right before my karta pobytu expires (btw Im not sure if visa and karta pobytu work the same), and comeback to get free 90 days visa. To do so I would go to Ukraine (Lwow) which isn't a Schengen Area.

However I was not sure If this method is valid, or have any risk in doing so. Also I was not sure If this method works legally, whether I could travel to Portugal and comeback without any problem within those free visa period. - Also I heard that South Korea and Poland have a free visa convention.

I have accommodation and I want to by flight to Portugal as soon as I get to know. Please help!
If you see any flaws in this methods of getting visa please let me know!

Thank you in advance