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How can I configure a headless Mac (MacMini, MacPro, MacStudio) controlled via Remote Desktop to match the remote resolution ?When no monitor is attached, macOS uses a hardcoded list of available resolutions, and you cannot change this resolution by the mechanism that SwitchResX uses. Thus you cannot modify this to match the Remote Controller's native resolution.
The only way would be to make the Mac think there is a monitor attached (using a dummy video adapter that simulates a monitor) - this one can then be enhanced with SwitchResX to match your requirements.
You can also use some active adapters (MiniDP+USB to HDMI adapter for example), that will simulate a monitor.
This kind of dongle/adapter includes some electronics and fools the Mac into thinking it has a real monitor attached, and then uses the signal to recreate a video signal.
When such adapters are used, the Mac doesn't see your monitor directly any more, it sees the adapter, and enables new resolutions (up to 1920x1080, or up to 4K for recent adapters or dongles).
Don't just use a pass-thru adapter, without electronics, which would require a monitor to be present. When it's a pass-thru, you still see the original Monitor name in macOS.
Then, you can add new custom resolutions with SwitchResX on the controller Mac, to match your remote Mac's available resolutions (you only require installing SwitchResX on the distant Mac to create custom resolutions, on the local Mac there's no need to create any resolution).