Hi @ Dan
Welcome to our forum!
Based on my research, you may follow the steps (File>Options>Mail>Stationery and Fonts>Personal Stationery) to set your default font in Outlook Desktop Client. And you could follow the article to set the default font for Word and PowerPoint. However, I cannot find a way to set up all M365 apps at once, please kindly understand that the Outlook tag here we are more relevant to general issues about Outlook desktop client.
Thanks for your support and understanding!
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