Microsoft have been removing control and ease of management from IT departments for years now with all their cloud products, and I don't understand why. Perhaps they're trying to reduce the breadth of features they need to maintain? I've got no idea, but it's frustrating as all hell.
I've been working on a solution that uses the same API that Outlook Online uses. The main problem is it requires interactively logging in as a user that has "full access" to all mailboxes; there's no way to assign the necessary permissions via an Entra app registration. Otherwise the API is pretty straight forward, which again makes me wonder why Microsoft don't provide a proper solution. (https://outlook.office.com/owa/$upn/service.svc?action=SetMailboxMessageConfiguration&app=Mail&n=64)
If you use your browser Developer Tools while authenticating with Outlook Online and then while setting a signature, you can get all the information you need. I then use the `PSAuthClient` powershell module to handle the OAuth authentication and go from there. I'm now up to finding the best way to give a single user account access to all mailboxes so it has permission to actually set signatures on everyone's mailbox. Ideally these permissions would be added and removed on-the-fly.