There are several old threads on the Samyang 8mm and fisheyes.
IMHO fisheye lenses can get a bit boring after a while. You get one, take the usual shots of a flying skateboarder, up the nose of a donkey/dog/child or of a building and you'll find that they all begin to look the same - fisheyed.
Although if you line up the horizon and are careful about the composition you can sometimes hide the distortion.
You might discover 360 degree panoramas - a whole new word. The Samyang 8mm excels at this since because of the unusual projection most of the time you only need six shots. With a decent software (e.g. Autopano) stitching is largely automatic and easy. And with a bit of care you can easily shoot 360 panos handheld.
- there are 360 degree viewer links on the Flickr page or try
360cities.
The lack of AF on the Samyang is largely irrelevant since at 8mm and stopped down a bit just about anything from a few inches away to infinity will be in focus.
De-fishing the Samyang is not that hard with PTLens and a bit of twiddling. You could probably also go the other way and 'fish' a rectilinear shot but it won't be easy.
Otherwise get the Pentax 10-17mm. At 17mm it is not at all fishy. And you get AF.
But since you already have an ultra-wide rectilinear you don't need that. If you do get the Samyang 8mm, you'll find that the extra 2mm plus the Samyang projection does make a great difference -
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/172414-fisheye...ml#post1826650