Every migration disaster I have been called in to fix had the same root cause: the redirect map was written after the design was signed off. Do it in the other order and migrations become boring, which is the goal.
Inventory before design sign-off
Every URL with a single impression in the last sixteen months goes into a sheet with a destination. If a destination does not exist in the new information architecture, that is a design decision to revisit — not a redirect to fudge.
One-to-one, or create the page
Blanket redirects to the homepage are an admission of defeat. They also tell search engines the old page is gone, which is exactly what you were trying to avoid.
The first fourteen days
Daily crawls, log review and Search Console monitoring, with one named person responsible for a 24-hour fix cycle. Most permanent losses are caused by temporary problems nobody looked at in week one.
