Approximately 2 months later, and I've finally finished retagging roughly 120 posts to include the new repeats tag. (I'm sure I missed some, but I am going to stop now.)
I would really like to apologise for continually restocking the active questions page with all of these old questions. I tried to keep it to 2-3 retags per day, and did my best to edit in other improvements to these posts at the same time.
However, I am sure other users have found this rather annoying, as it has pushed other (more worthy) questions out of the active page quicker than usual.
I can totally understand this objection, but apparently there really isn't any other way to get old posts retagged.
On Meta, I posted this question/complaint/rant which summarised this problem and documented a list of related questions:
The system for adding a new tag to old questions is broken
but it hasn't received a proper response yet.
Overall, I've found the process to be very tedious, and I'm not really sure if it was worth it.