Tried with the public beta on another machine (intel) and it works (its usable) but a bit laggy. Also did a full install on the original Mac mini (M1) and its now usable.

Teams and Outlook both working in beta 5 on M1 mini

Its an HDMI connection between the mini and the monitor and wireless mirroring when connecting to the MacBook. So I know the monitor can handle the resolution as it does it when replicating the resolution from the MacBook but when disconnected its not an option. I've also tried easyRes and SwitchResX but neither allow me to add the custom resolution to the mini. Thanks

Seems to be an icloud sync issue. Logging out of icloud on the mini and logging back in (to icloud) has resolved the issue.