As you say, the options are all poor when compared to the US/UK but your best bet right now is probably to pick up the no-fee version of the mBank card. The rates were devalued in January (from 5 to 6PLN per mile; 3 to 4PLN for the premium one). If you think you can get enough spend on the card it might be worth considering the premium one for a higher return.

I tried getting one of the mBank M&M cards recently but got rejected. I'm a freelancer and Polish banks REALLY hate self-employed people! If you have a full time job in Poland (umowa o prace) it should be easy. Otherwise, expect the bank staff to set the dogs on you.