Was Poland ever part of Soviet?
Hmm... that seems more like a distinction without a difference.
Though dominated by Russians (mostly... Stalin was a Georgian), the USSR was made up of over 100 different nationalities with widely varying languages and cultures.
Totally different things.
I know its not trendy, but its probably easier to think of the eastern bloc countries as colonies of the USSR, just as india, egypt, and the african nations were to the UK.
Sure, on paper they were sovereign, and they certianly had a government made up of people from that country. But they were a colony.
It's not clear that the term colony implies direct governance. For example, the Brits applied almost no direct governance to their colonies in the Americas from 1650-1750.
One could also just say that the Russians occupied Eastern Europe for the half-century following World War 2.
It was more like the Saxon colonisation of the UK, which replaced the native Celtic population, rather than the Norman conquest, (or the colonisation of India) which just replaced the oligarchy.
What came quite a lot later in America , was the attempt to assert more direct Imperial rule over the colonies.
They were countries conquered and occupied by USSR troops, with USSR military bases in them to guarantee obedience. There were local rulers, sure, but major decisions were consulted with Moscow. Many decisions just were out of question, because it was obvious that Moscow would not approve and it may end in a military intervention (as in happened in Hungary and Czech Republic). So no, I would not describe them as "sovereign countries". The proper term is something like "puppet state".