How about overlapping thread lifespans? This way when a new thread is created, recent comments on the previous thread won't go unread, and discussion can still happen there. A thread on Monday that lasts a week and a thread on Thursday does too, for example, with both threads pinned to the top and included under the Latest Open Thread feed on the side. I suspect this would be easier to implement than your second option. It's more difficult to implement than your first and third options, though.
Overlapping threads are just an agreement of everyone to keep looking at old posts. This is harder to orchestrate, because it requires lots of people to change.
This week's open thread is less than a day old, and has already accumulated more comments than the 15 latest non-open-thread posts combined. I fear the thread will wither and die before Friday.
Going from monthly to weekly open threads was a big hit. Should we ratchet up open thread frequency even more? Should we add more outlets for comments, or will comments inevitably expand to fill the available room?
Proposal for discussion: We follow a regimen of weekly blather threads for the next two weeks, then reassess.
Addendum
I posted a Links on Friday and a Stupid Questions on Monday. In the 5 post-days that they've been up, they've accumulated 32 comments. Based on these numbers, it seems unlikely that these topical open threads will relieve pressure on the main Open Thread and so I've stopped the experiment.