Anamorphic mode is not for anamorphic lenses.
What could help is crop mode, if there would be decent 4:3 preset then you could iuse some 2/3'' sensor lenses with your anamorphic lens x2 to get similar FOV to what you have now and maximum resolution .
crop modes are optimized for 16:9 and cinemascope ratios, You can trim sides from 16:9 but its not real maximum 4:3, its just not many people need it
In theory 2200x1790 could be possible if you would take sides from 2.8k and applty them to top and bottom resolution while reducting left and right, Not sure if that would help but it might be the highest 4:3 resolution this camera could handle at the moment and this also could help us get nice 16mm film emulation preset in 4:3.
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Ah You have 5D3... i thought it was M.