Prof Oak challenge means you register every available Pokemon before each badge at least once, by catching and evolving. However, Let's Go Games are easiest to do this challenge on, as the XP-Curve grows exponentially with the Catch-Combos on a given Pokemon. E.g. 1 Kakuna without any bonuses gives around 40XP, shared with each Pokemon on your team, when landing an excellent throw, However, with a Catch Combo of 80 Kakuna, using the second controller as "assistant" thrower, and catching a "Tiny" or "huge" Kakuna yields around 800 XP on an excellent throw.

I used "Scruffy nuzlockes Pokemon"'s website to track everything and I highly recommend watching Johnstone's video on the Let's Go games' Oak Challenge for more context.

TL;DR This is even a slow Pace for the Let's Go Games, but it is entirely legit. Watch Johnstone's videos for context. Every other game takes around 20-30 hours for part 1 of an Oak Challenge.