I have never seen a migration lose significant traffic for a mysterious reason. It is always the same short list: incomplete redirects, changed content, or a staging site that went live with noindex still switched on. All three are preventable with a checklist and someone whose job it is to hold it.
Four weeks before launch
Crawl the current site fully and export every URL with its traffic, rankings and inbound links. This inventory is the single most important artefact of the migration, because you cannot preserve what you have not listed. Identify your top pages by value, not by opinion.
Two weeks before
Build the redirect map: every old URL to its closest new equivalent, one to one. Where there is no equivalent, choose deliberately between the nearest category page and a considered 410. Redirecting everything to the homepage is the classic shortcut and it wastes almost all the equity you were trying to keep.
Then run content parity checks on the top pages. New designs routinely trim copy for aesthetics, removing the text that was doing the ranking. Compare word count, headings and internal links, and put back anything that disappeared for visual reasons.
Launch week
Audit staging for indexability, canonicals, schema, speed and tracking. Confirm the noindex tag is removed at go-live, which is the most common single-line disaster in this whole process. Have analytics and Search Console ready for the new property before switching, not after.
Launch day
Push the redirects with the site, not afterwards. Crawl the live site immediately and fix any redirect that returns the wrong status. Submit the new sitemap and use the change of address tool if the domain moved. Verify tracking is firing before anyone celebrates.
The four weeks after
Monitor indexation, rankings and traffic daily for the first fortnight, then weekly. Expect a brief wobble of one to three weeks; that is normal. What is not normal is a decline that keeps deepening after three weeks, which means something in the map is wrong and is still findable at that point.
Done this way, migrations are routine. Done without an inventory, they are the reason a business spends the following six months trying to get back to where it started. Migration support exists precisely because this is a checklist discipline rather than a talent.