Since the native migration option by Microsoft only takes care of the migration of the slack channels and files attached to them, it skips 1:1 messages and group chat entirely. In case these are important to your team, you have no choice but to use an external migration utility which is specifically designed for such a task.

The main points of consideration while looking at various available tools would be whether it maintains a threaded structure of your messages with sender names and timestamps, or whether it imports them flat; whether it maps Slack users to Teams users automatically based on email address correspondence, providing a manual mode in case of discrepancies; whether it can perform incremental migration, meaning it won't re-import the data already migrated in the previous import process.

Kernel Slack to Teams Migration Tool is one of such utilities, it imports 1:1 and group messages into the corresponding Teams chat along with public and private channels, maintaining original message threading and sender/receiver identity. Batch migration and date filter features may also come in handy in certain cases like migrating particular period only due to compliance requirements.

Read More - https://www.nucleustechnologies.com/slack-to-teams-migration/