Most people I know have Arts or Humanities or Law - and none of them are doing those things.
Classic case of sampling bias. Also, no one here on PF seems to appreciate the value of humorous hyperbole.
If I had my time again, I'd do Dentistry or Law
Dentistry opens doors, though not for a foreigner in Poland. Law doesn't, unless you have good connections. Most people who graduate from law school never work a single day as a lawyer, both in Poland and the US. It's a notoriously difficult profession to break into, and in both countries, law schools produce far more graduates than the market can absorb.
I'm happy with my two (one of them non-Maths based) degrees
Have two humanities degrees myself. Yes, they make you richer person spiritually, mentally and culturally, but I'm glad I have my science and medicine degrees to butter my bread, as well.
By the way, I don't consider marketing, media and communications as "humanities" or "liberal arts". They are too vocational for that.
follow it up with Marketing as a Masters, further own the line.
The OP is asking about masters programs, not undergraduate programs.
maybe time to do a (perhaps non-work related) degree in an intellectually rigorous subject
Which is more or less what I suggested. Watch this brief speech by Guy Kawasaki for similar advice based on the regrets of what most people consider a fabulously successful man:
youtube.com/watch?v=4PSR09ZSfTY