The non-negotiable deliverable is a one-to-one redirect map built from a crawl of the old site plus every URL with impressions in Search Console and every URL with external links. Anything left out loses its history. Redirecting non-matching URLs to the homepage is not a fallback; engines treat it as a soft 404 and the signal is lost entirely.
Plan the measurement before the launch: a frozen baseline of impressions, clicks and positions by template, a crawl of the old site kept as an archive, and a monitoring window of at least eight weeks. Expect a dip regardless of quality — the question is depth and duration, and a well-executed migration recovers within four to eight weeks.