While I don't specifically have an Express of any model, I have used Miracast without issue to my Ultra from a Windows laptop. Years ago I did have an Android phone, and I remember I had to have an app to enable screen sharing on my Ultra 4640 at that time. I don't know if that has changed with newer versions of the Android OS.
For Windows, remember that the Miracast protocol requires the Windows PC to have a WiFi card, so a basic desktop PC will not be able to screen share to a Roku device. My Windows 10 laptop works just fine casting to my Ultra 4800. I recently received a Windows 11 laptop, and I have not tried screen sharing it to my Ultra. I'll try to remember to test it this evening.
Dan
Roku Community Streaming Expert
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