I used the KP with the DA* 11-18mm. Shot in RAW Processed in Lightroom 6

Despite living in Suffolk I had my first trip onto Orford Ness a couple/few weeks ago. A very unique and strange place.

Keep on practicing and be prepared to edit your images differently to any usual habits you may have for sharp images. Good advice already given. It's not always about a lot of movement but choice of movements. As said trees are a good starting place.

The Sigma 150-500mm would certainly be your cheapest option/ only option to get to 500mm. Lots of people rave about the DA*300mm which they say works well with the 1.4 teleconverter.
I have the Sigma 150-500mm bought new and used on the K-7, K5 and now the KP. I had to send it to Sigma UK and they updated the firmware FOC so the lens would work with the KP, I can't say if it would work with the K3III. Sigma UK were very helpful, I'm sure they would tell of its compatibility.
To be honest I hardly ever use the 150-500mm and probably ought to sell it to someone who would use it. If you decide to go Sima direction I'd happily talk about selling the lens.
Nigel

Very sad news, my condolences to his family

Long may it continue so we may see more great cameras and lenses.

I must admit the offer was very tempting but, I managed to resist.
Maranatha enjoy your new camera

What them others said
And, so cool walking around with this lens on your camera

I'd be surprised if there were any updates to Pentax wide angle APS-c offer anytime soon but, who knows!
I believe Irix and Samyang have wide angle offers in Pentax mount, could be worth looking at.

That sounds like quite a deal for anyone interested in the lens.