sigma fp l + SIRUI 35 mm T2.9 Anamorphic : A process for de-squeezing and lens correction

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Hello!

We use a combination of sigma fp l + SIRUI 35 mm T2.9 Anamorphic (https://www.siruishop.de/en/products/venus35) for video and photo. Now I am writing concerning the photo application. The process of de-squeezing in Photoshop and the following lens correction process seem to be non-optimal. After 1.6x de-squeezing in Image Size the image, the image is distorted at the right and left sides. So we correct the lenses in Photoshop (Filter - Lens Correction - Custom - Remove Distortion). If I set it to ca. -15/-12, then the verticals are fine, but the horizon is curvy. And if I let the Geometric Distortion 0, then I have the horizon straight, but the sides are distorted. So the question is - what do we do wrong?
If I change the sensor setting from 3:2 to 16:9, the same thing happens. (BTW What does it bring, to switch from 3:2 to 16:9 using this lens?)

The second thing: we do need the image size of 2.8:1 or at least 2.4:1. If this SIRUI 35 mm T2.9 Anamorphic doesn't offer an image with proper optical proportions, would be a solution to take a Sigma 35mm F1,4 DG HSM | Art (https://www.sigma-foto.de/objektive/35mm-f14-dg-hsm-art/uebersicht/) and just crop the 16:9 image to 2.8:1 or 2.4:1?
Thanks in advance for your answers! 

BR + thanks,
Tim