Collecting Evidence of Slack Messages

My former client CEO doesn't want to pay me an invoice. Proof that he is liable is found in direct messages between us on Slack. I'm not sure he is aware of it and so far we haven't gone to court. Slack just informed me that the CEO, who is the workspace owner, can delete the whole workspace. I asked them whether screenshots and video recordings of the messages is admittable evidence in court, or if there is something else one can do. Didn't get a clear answer, anyone have experience of this and how best to secure the evidence?