Feel free to spin up a Discourse for Teams trial and enable the slack integration, then see what you can do to save transcripts of valuable conversations in discourse. Transcripts are all saved in a single post.
You can create an invite link and post it to your slack to ease the transition. Ideally you’d then set a deadline for new conversations to start happening on Teams and close the channels on slack except for ephemeral contact, as Rafael and Justin are saying.
Note there is nothing public in Teams - if you need public discussions you will want to look at Discourse, not Discourse for Teams. There are also other differences. See Comparing Discourse for Teams with Discourse.