Output SDI MONITOR Aspect Ratio issues

The problem is not if SDI or HDMI, the problem is when using an external video card, the only way it goes out ok is using a direct HDMI from the Mac (using a dongle or cable) but this uses the internal GPU and the point of using an externa video card is to avoid that.

Hi all,
Three years later and the problem with A/V Output on Final Cut X 10.5.2 with non 16:9 aspect ratio still the same ! Or ....?

Yes, this is unchanged as of the current build. I run a small editing facility with 5 FCPX seats, and since 2017 (when we started working on 2.39:1 and 2:1 shows), this issue has forced us to work either with desktop overlays on client monitors, or create playouts and show those in Resolve for client sessions.
FCPX performance and stability go down drastically when creating a 16:9 timeline with a blanking mask.
Audio sync and playback performance with multiple monitors hooked up is patchy at best - we have tried this with everything from iMacs/Mac Minis up to the current model Mac Pro, but A/V sync is always bad, and starts drifting even more a few minutes into playback.
Using a Blackmagic/AJA card still results in the viewer image being stretched to 16:9, and never mind the quality compromises associated with having FCPX stretch your footage and then using your TV/broadcast monitor to squeeze it back - lots of monitors simply don't offer settings to do this for 2.39:1, and I know of none that can do it for 2:1 content.

The only workaround we have found is using a Blackmagic ATEM's DVE to squeeze the image back into shape - but adding another piece of gear to the signal chain does nothing to make it less error-prone.

If anybody has a better workaround, or has a contact at Apple who might be able to finally get to work on this (we've been pestering Apple Germany about this for four years), I'd be thrilled to know.