Every time I try to play with Haskell I give up in just a few days. Too much math! Maybe you don't need a PHD, but from my perspective you do need more than an undergraduate in Engineering.

So, what's a good purely functional language that's easier to learn than Haskell. One that I can learn more by hacking about than trying to keep so many things juggling about in my memory all at once?

Ideally one that runs native on Windows and Linux and is suitable for web and business apps? So I might actually end up with something I could put in (light) production.

OO has 'Inheritance', 'Subclassing', 'SOLID', 'Liskov', every design pattern name, 'Object', 'Class', 'Overloading', 'Factory', etc, etc.

Haskell has similar words, covering both core concepts, and common patterns of implementation. 'Monoid', 'Monad', 'State', 'Reader', 'Function', 'Lambda', 'Typeclass' are all examples of terminology in Haskell

None of those words are inherently more difficult than the OO ones, you're just not familiar with them.

The best way to learn Haskell is to follow along with a course that has you do exercises, and actually do the exercises. See the https://github.com/bitemyapp/learnhaskell course for some class recommendations.