The risk to that approach is that you end up writing code that cannot deal with the real world problems of I/O, such as timeouts, failed reads, jitter, and other weird behaviour.

Separating I/O from logic makes a lot of sense and makes tests much easier to write and code much easier to reason about, but you'll still need to implement some sort of mocking interface if you want to catch I/O problems.

> how can I take the 90% or 95% of this function that is pure and pull it out, and separate the impure portion (side effects and/or stateful) that now has almost no logic or complexity left in it

They addressed this concern already. These are not contradicting approaches.