I am worried that an optimisation here complicates stuff enormously for very little benefit.
Think it through …
Say there are 6 chunks on meta. If a topic from the last chunk is touched… the entire chunk becomes invalid, you got to remove the topic from there and put it in the front chunk.
Optimising here is a little pointless for a site that sees any kind of activity and the dates inside the chunk on the actual topics are fine.