macOS 14 or later
Decky
The Mac that gives
€59 one-timeThe menu-bar app that makes displays. It creates the screens, encodes them, carries the sound, injects the touch and the pen, and decides who is allowed to do what. This is the one you buy, once.
It makes the screens
Spare desktops with nothing behind them, displays for a tablet over cable or WiFi, and Rooms that reach every device at once.
Every hard part lives here
Creating a display macOS believes in, the USB accessory handshake, screen capture, hardware encoding, event injection and the audio driver are all on this side. The clients decode and send input back.
Not on the Mac App Store
It uses private display APIs and claims a USB interface, and either one is a rejection. It is sold directly, signed with a Developer ID and notarised by Apple.
Bought once, then it stops asking
The licence is checked on your own Mac against a key inside the app. No activation call and no periodic check.
The free version has no time limit. The paid features simply are not in it, so there is nothing to unlock.