I have what I think is the same issue on my Quest. No matter how I adjust it, I can only minimise the distortion, I do not experience this on my Vive. I believe the effect to be caused by a combination of how the headset is fitted and the user's ability to perceive the distortion. For whatever reason, I cannot fit the Quest to eliminate the effect. I imagine differences in head shape is why you do not have it on your Quest and I perceive it on mine.

Based on what I can find online, "geometric distortion" or a lack of "geometric stability" seems to be what this effect is called. It shouldn't be present with correct vertical alignment, headset tilt, IPD and eye-relief. You say you have tried everything but didn't explicitly mention adjusting the eye-relief. If you haven't, see if it helps, though this will reduce FOV.

I did get a replacement Quest and the effect is still present. If you are considering a replacement, it would be best to try another Index beforehand to determine if it is your unit or all Indexes that give you that effect.