Sorry, this Workflow will work up to Catalina only. Apple made big changes to the menu bar system with Big Sur, and basically another script would have to be written (and researched properly) to make it work with it. Free free to fork or PR or cry hard.
Rather than clicking the menu bar and and choosing your display and blah blah blah, just open Alfred, type aps, choose your display and KABOOM!!! Your display is extended to your AirPlay device.
P.S. You can now access your Sidecar devices easily with the new Go to AirPlay Menu option. 😎️
SETUP NO MORE! AirPlay Spreader now detects your AirPlay video servers automatically. (Only if connected to the same Wi-Fi network though.)
If you're using Bartender, hide the AirPlay Displays item and extend your display, you will not be able to Turn it Off from the Workflow. The current workaround is to Show or Always Show the AirPlay Displays item in Bartender. If you're mirroring your display then you don't need to do anything, it just works.
Latest release available on the release page.