Back in the 90's, I worked at Vodafone doing C++ and the tech leads were two Smalltalkers -- and I learned so much about OOP from them in just a few months! I've also done some Smalltalk myself and I'd highly recommend it as a way to really learn how OOP was intended to work (before C++ and Java poisoned the well).

I never used it but maybe it is worth trying https://pharo.org/

When I was studying at University of Warsaw we had a lecture OOP and the lab and they introduced OOP with Smalltalk