Is there anywhere to see which policy settings (meetings, messaging, calling) we should set for a user if we set TeamsUpgradeMode to SfBOnly or SfBWithCollab respectively? Or is it only the warning message you get in Powershell that direct you?
Have you tested having users in SfB Only mode, setting all policies to AllOff and assign them the Teams license? Anything to look out for in that setup? To me it looks like it will give them the opportunity to be part of teams but still be using SfB for chat and meetings.
Do you happen to know if there is a way for SfB users to attend a meeting being signed in with their account? Currently they need to sign in anonymously meaning that they get to the lobby and that they don't have access to meeting notes.
The thing stopping them is that they don't have a Teams license. It would be ok to assign them the license if we could allow them to only attend meetings with the license and not use Teams in any other way.