AMEX acceptance is pretty common in more touristy areas etc., but sometimes it is lacking in random places. My understanding is that Amex is the most expensive for merchants hence they opt out. There are no banks issuing Amex, so it is not possible for a Polish person to get one without going to a foreign bank. But still, I would say maybe it's at 70-80% of merchants were Amex works, I could be off. But yes, that is the only sure way to avoid DCC, although like I said, it is pretty easy to reject it in almost all cases.

As a side note, Discover is also supported in about as many places as Amex is. I am actually quite happy as I get to get 5% back on restaurants during this trip while having no FTF