G'day,
I know it is normal to consider wide angles for landscapes, but telephotos can also do a brilliant job too. I suggest you drop by this thread:
Post your Telephoto Landscapes! - PentaxForums.com and keep some longer lens options in mind as well.
But of course wide options do give you the grand vista landscapes and I am a fan of these too. And if you're up for getting into panorama images then normal and short teles become great options when the subject fits. But as you've asked about a wide angle lens option here's my 2c.
There's unlikely to be any 8mm options in K mount FF. The only one I"m aware of is a Sigma 8mm circular fisheye that you may struggle to find in K mount:
DSLR Lenses, SLR Lenses | Camera Lens | B&H
To go ultra wide the Pentax 15-30 is a great option to keep up your sleeve. The WR and zoom range means it's a flexible option to consider. The filters for this lens will cost you money though. They require a 150mm filter system which means it's not cheap. There are options for this lens available from Benro, Lee, Nisi, Fotodiox and others so if you want filters for the 15-30 you can get them you just have to factor the costs into going with this lens option, or adopt a PP approach to images that would normally require filters.
I'm surprised that nobody has mentioned the future release of a Pentax wide angle prime, but there is apparently one coming. Breath holding is not recommended for this one though so to get back to the here and now if you are looking at really wide options then consider the Irix 11mm and the Laowa 12mm. Both appear to be viable options and are likely to get you as close to 8mm as you can on K mount. To get a feel for them and other alternatives have a look at this thread:
ULTRA wide lenses (only FF equiv 12mm or lower) - PentaxForums.com I hope to get the Laowa 12mm at some stage, the zero distortion moniker should mean a good architecture option and less image space lost due to distortion correction, a problem out this wide (hence my earlier pano suggestion). However refer to posts by todd in that thread as he's found some issues with getting set up with the Laowa though now he seems happy with the lens he's got after returning the first one.
For more examples of the Irix 11mm and Laowa 12mm, check out these Flickr groups for each of these lenses:
Irix lens and photos | Flickr LAOWA 12mm F2.8 ZERO-D | Flickr IRIX 11mm f/4 Firefly & Blackstone | Flickr Laowa D-Dreamer 12mm f/2.8 (ZERO-Distortion Ultra-Wide Prime) | Flickr
Good luck with it mate.
Tas