"Back in a days it was a lot, lot slower and more detailed."
Oh for sure, I have childhood memories of the really dark time, but it is way worse now, than it was back in 2015. I missed flixbus connections because of intense checks and changed vacation plans avoiding long waiting times at borders within EU (my recommendation, cross at night).
Every country can choose independently, and for each border. From a cursory
search, it appears e.g. Poland currently only checks the borders with Germany and Lithuania. You'd have missed any checks on your specific trio, if this list was true at the time:
Oh for sure, I have childhood memories of the really dark time, but it is way worse now, than it was back in 2015. I missed flixbus connections because of intense checks and changed vacation plans avoiding long waiting times at borders within EU (my recommendation, cross at night).