Data migration is where most CRM projects lose time. The work starts in discovery, not in the week before go-live.
Begin with an inventory: which records exist, where they live, who owns them and which are still used. Archive what nobody needs.
Map every source field to Dataverse, agree how duplicates are resolved, and clean the data at the source before moving it.
Rehearse the migration at least twice in a test environment, reconcile the record counts, and have users check samples before the final run.