The migration tool is nice because it allows you to select a date range for import.
Either way you should ensure that there are no slash “/” characters in folder names in Outlook as that wreaks havoc with other clients parsing of the label names and can create duplicate folder trees one version with a trailing slash one version without.
Also recommended is moving the folders that the user has inevitably created in the inbox into the main tree of the directory structure. Since labels in Google are limited to 42 characters keeping names short helps. Pair with the client and see if there are any trees that can be renamed BEFORE migrating since it’s easier to change a proper structure than to attempt to fix the labels once they’re in Google.