The
DFA 24-70mm seems like a great lens, but it is not really very wide by modern standards. Something like
DFA 15-30mm might be better suited for ultra wide and it would still give you a comfortable zoom range.
That said, I prefer primes. If budget was no issue, I would look into Zeiss ZK in the range between 20mm to 28mm. Never had a western, modern Zeiss lens, but they seem to be pretty good
Since budget usually is an issue, I would look into Samyang 14mm (have it, love it, FF lens, but I use it on APSC) or Samyang 24mm. Or better yet, wait for the new Samyang 20mm to be released.
Unfortunately, Pentax is currently missing Wide angle primes. The widest you can find is the superb FA 31mm ltd, and it may be wide enough for many things. I would prefer to go with 28mm (like F 28mm f2.8) or wider, because I usually shoot wide angle.
Just my tips, subjective as they might be. You can check my gallery (sig) to see which primes I use for what kind of photos, but even though all my primes are FF, I use APSC camera