Abstract on Netflix isn't a documentary series, but a few episodes walk you through real-world design decisions. I found the Ilse Crawford and Tinker Hatfield episodes particularly useful - they show exactly the kind of process and trade-offs you're after.
For something closer to a proper rebrand case study, Pentagram's YouTube channel has detailed breakdowns of major identity projects. Not documentary style, but the thinking behind each pivot is laid bare.
If you're documenting your own work, skip the big reveal. The real story is in the client pushback, the rounds that got binned, and the brief that got rewritten halfway through. That's where the networking gold lives too - understanding how relationships shape the outcome.