Ooh, thanks for these suggestions!
Aleksandra probably wouldn't work with
Oliwia, since it's inevitably nicknamed
Ola, which is rather too similar.
Anastazja could work, it's very dramatic. It was the name of a Ukrainian classmate of mine back in middle school.
Urszula sounds very serious and solemn to me compared to
Oliwia, which is seen almost exclusively on under-25s.
Ula is sweet, though.
To be honest, I don't really like
Krystyna. It's considered dated, and I only know one under 40. She goes by
Krysia, which, unfortunately, is also an inevitable nickname here. It's pronounced KRI-sha, which I thinks sounds thoroughly unpleasant.
masculine list:
https://www.behindthename.com/pnl/191050/124079feminine list:
https://www.behindthename.com/pnl/191050/124080