While Smalltalk does have higher order functions, it’s not really a functional programming language. It’s a pure object-oriented language through and through! Of the languages you’ve used, only Delphi has object-oriented features, and even then it’s a something of a mishmash. Perhaps you didn’t truly embrace OOP and that may present a learning obstacle. It is commonly accepted that the best way to learn OOP is to learn Smalltalk. Not C++. Not Java nor C#. Not Scala. Not even Ruby.
Some of the best learning resources for OOP and Smalltalk are: