Migrate site to new host

We need to move the site to the new host this weekend with minimal downtime. Export the database, copy the wp-content folder, update wp-config on the destination with the new DB credentials, replace the domain’s DNS A record at 02:00 a.m. Sunday, and test the homepage, blog, and checkout before confirming with the client.

Before I hand the migration task to the junior admin, make the package easy to approve: produce a short checklist with estimated downtime, the exact DNS record changes and TTLs, a sync plan for uploads changed during cutover, and list three things the reviewer should test immediately after DNS propagation.

I copied files and updated the database on the new host, then changed the A record as planned, but after propagation the site still loads assets from the old host and some links break. I suspect serialized URLs or caching. I can’t explain this to the CTO without sounding inexperienced. Do I run a search-and-replace for URLs, purge caches, or roll back DNS? Which step most likely fixes mixed-host assets with lowest risk?

Across five migrations this quarter I keep underestimating time for media sync and DNS propagation, and clients are frustrated by perceived downtime. Where am I miscalculating — sync method, TTL strategy, or preflight testing — and what single change to my migration routine will reliably cut downtime and client calls?