This is messy with 20 episodes dropping all at once, but option #1 is probably the best course of action.
Another idea: All 20 discussion threads, but posted two per day for ten days. Then after two-ish weeks, posting a season-long discussion thread.
Regardless, I like the idea of a new "So what did y'all think?" discussion thread posted a couple of weeks later for those of us who don't prefer to binge.