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Very helpful!I don't think there's a Samyang 7.5mm f/2.8 - there's an 8mm f/2.8 and a 7.5mm f/3.5; the latter only being offered in MFT mount. The 7Artisans 7.5mm f/2.8 works reasonably well in that dome, with the caveats of fixed aperture and manual focus. Focusing is possible with the zoom gear from 16-50mm lens and some padding; I posted pictures earlier in the thread. Focus peaking helps immensely. Vignetting is slight and easily corrected. Sony 16mm f/2.8 with fisheye converter should also work well, with AF and aperture control.
Regarding flat vs dome - there's plenty of articles that you can Google by the terms 'dome port theory' which explain it in depth, but the gist of it is, light rays hitting a water/glass/air boundary at an oblique angle get refracted and scattered, which causes image quality degradation, increasing with the angle. Past a certain angle, you can't see anything at all. Dome ports solve this issue by presenting the light that enters the port with a boundary surface that is always perpendicular to the rays' direction, or nearly so.
Avoiding scratches - the dome ships with a neoprene cover; best practice is to only take it off after you reach your planned depth and level off, then put it back on at the safety stop. Never leave the port uncovered while entering or leaving the water.