Honestly, I think that's the wrong way to look at it. It's not a "huge undertaking", you're under no obligation to read the entire canon at all, let alone to do it in any particular timeframe. You can take a break, or just stop entirely, any time you like.
I read the books pretty much as they came out, which means that even at the peak there were gaps of up to 6 months between books. That's how they were written. I think it's a huge mistake to try to devour them all to some arbitrary self-imposed deadline.
Publication order is the best way to see "how he developed as a writer". Viewing them as separate "series" is always slightly misleading, because there are crossover elements between them - the only real advantage is to make it more "approachable", and that's only an issue if you're thinking in these "undertaking" terms.