1. Naming is hard... sqr -> sqr_as_str
2. That's absolutely not OOP, here ;) More like functional (except for the cache paart).
3. Overengineering is fun, yeah (I like that too). But one has to stay reasonnable. Reinventing a string iterator is pointless, for example (aka, exit gen_digits)

for juniors, to get known about new syntaxes and thoughts.

for seniors, to warm up and recap their knowledge at the start of the day.

for me, i used OOP method. im junior tho, but want to get used on OOP.

why use the complex method to this kata? What does it means?

thinking about scalability from the first minute, i like it

I love the caching angle (with a cost for short numbers, though) but not sure gen_digits is an improvement over simple str() which returns an iterator out of the box.