If you're planning a Slack to Teams migration, I’d recommend starting small instead of trying to move everything at once.

For a small workspace, you can manually recreate Teams channels, download important Slack files, upload them to Teams, and copy over critical messages or information. Before migrating, it’s also worth cleaning up unused channels and deciding what data actually needs to be moved.

The main issue with the manual approach is that it gets difficult pretty quickly when you have lots of channels, DMs, files, and message history. User mapping, attachments, replies, and older conversations can easily be missed.

A better approach is to test the Slack to Teams migration with a small group first, verify the results, and then move the remaining data in batches. Keep the original Slack data available until everything has been checked.

For larger migrations, a free tool can save a lot of manual work. Kernel Slack to Teams Migration supports migration of channels, DMs, group chats, files, attachments, and other Slack data, with options such as user/channel mapping and selective migration.

You can check it here:
https://www.nucleustechnologies.com/slack-to-teams-migration/