Samyang 14mm is rectilinear and it is FF compatible. It is great in that respect, and very sharp. But it has its downsides. There is some distortion on APSC, so I assume it becomes more apparent on FF. You can use a lens profile to correct this in PP. But it also depends what kind of shots you are taking - sometimes the distortion will be more apparent than others, depending on the subject.
Other downsides are QC (miscalibrated distance scales, decentering) and lack of filter threads (and the lens cap, I guess. Its a bit awkward, but that is not so unusual at this lens size).
I don't have experience with the other lenses, but yes, those are the main FF UWA candidates. You can also look for Zenitar 16mm fisheye, or some of the legacy UWA lenses. There are a few in the range from 15mm to 25mm, from Sigma to Zeiss to Voigtlander and of course Pentax.
Note that FF will give you a wider FoV already, so even 35mm will be wide angle. 28mm even moreseo, and 24mm begins UWA range. So even something like Samyang 24mm f1.4 or Sigma mini wide or Pentax FA 24mm can give you really wide FoV. Sometimes people get obsessed with ultra ultra wide, but then once they get it, they notice how many limitations it has. I don't know how much experience OP has with UWA, but I would recommend starting with 28mm, 24mm. On FF, that is plenty wide.
Edit: And isn't there a Tamron 10-24mm? Not sure at what focal length it covers FF, if ever. Here is a comparative review of APSC UWA zooms:
https://www.pentaxforums.com/reviews/pentax-12-24mm-lens-comparison/introduction.html?src=lrdb
Some people buy Samyang 8mm, which is an APSC fisheye, then remove the hood so the image circle is as big as possible. This modification is especially popular with Canon users, I think. Feel free to search for it
Irix and Laowa are going to release some ultra wide Pentax lenses soon. 15mm macro, 12mm with zero distortion. Definitely interesting lenses, not sure if they are available yet, but feel free to preorder. Irix comes in two versions, but with same optics.
Of course, you can also look at Ricoh Theta 360 for.. not only ultra wide, but ultra widest! As it captures 360 degrees, everything. But yes, get that Samyang 14mm. It is very sharp and quite great overall, just make sure you get one with warranty and double check for decentering. Samyang lens club here:
https://www.pentaxforums.com/forums/122-lens-clubs/137335-samyang-lens-club-38.html
Many samples of various Samyang primes, but most of them from APSC cameras