Okay, there were some serious misunderstandings here in this discussion.
First, actually Poland has double tax treaty with US.
Second, it is very unwise for Polish tax resident to cash out cryptos for himself if he wishes to avoid tax.
Solution is relatively simple - one has to change his/her tax residency to a country which will not tax him/her on the crypto income. There is actually no need to create an offshore company to bypass the funds. The only idea I have about this advice is that lawyers are afraid that previous crypto trade will be allocated to Mark because under Polish law there can be different interpretation of moment of creation of tax liability towards state. And that is why they are proposing offshore company - to pretend that it was traded not by Mark himself, but via offshore company.
This may also not work for some reasons (e.g. tax avoidance clause in Polish tax code and mainly because Mark was Polish tax resident during that time). So from my perspective it will not give Mark any extra layer of protection. So, we go back to the simple things - we need to switch residency. Why? First of all, because any legitimate bank (let's say Barclay's or other private bank) has to comply with CRS. Under CRS they will report bank account UBO to the country of residency. Because Poland is well known for its irritating tax authorities it is better not to cash out as a Polish resident.
Somebody advised a Malta here. I am not very into Maltese law actually. My idea would be UAE residency and bank in Georgia.
Maybe this will help Mark:
Czy istnieją jeszcze prawdziwe raje podatkowe? - Prawo do prywatności