(I take advantage of the fact that @Andrew is sleeping to hack his diary )
You mean that in Polish there is also the difficult and rare sound of “multiple r” or “strong r” of Spanish? Like this one: HOW TO ROLL "R" IN POLISH // ItsEwelina - YouTube?
If it’s the same sound I had trouble pronouncing it too when I was a kid, but it was enough going to a speech therapist. I still remember some of the exercises/tongue twisters aimed at improving it! Like:
“Erre con erre guitarra,
erre con erre barril,
rapido ruedan las ruedas del ferrocarril”
(“Erre con erre guitar,
erre con erre barrel,
fast the wheels of the railroad roll”)
This is what a similar version sounds like: Tongue Twister erre con erre - YouTube