Whilst there are all sorts of benefits folks will legitimately (or otherwise) claim for shooting full frame, there are two that I find particularly relevant to my own use cases.
Firstly, there's the opportunity to use older manual glass designed for 35mm film SLRs on the format for which it was originally intended. Using vintage SLR glass on an APS-C body is all well and good, but you don't get to experience it in the same way, since so much of the image circle is cropped away. With a full frame camera, you'll experience every part of the projected image that the manufacturer intended, warts and all. It's not necessarily
better, but it is certainly very
different, being nearly identical to what you'd see on 35mm film. As such, it also provides a greater number of lens choices for wide-to-moderately-wide-angle photography.
Secondly, there's the opportunity to shoot with shallower depth of field for any given field of view than is typically practical with APS-C. When you shoot, say, a 50mm lens on full frame at a reasonably modest aperture of f/2 for the first time, you might be amazed at the sense of "space" and "separation" in your images (not a great way to describe my point, but I can't think of better... hopefully you get the idea). I don't want to turn this into yet another so-called "equivalence" debate, but with APS-C you'd need something like a 35mm f/1.4 lens to achieve a similar look... and if you should shoot that same 50mm lens at f/1.4 on full frame, well... there simply isn't a lens around that will give you a similar field of view / depth of field combination on your Pentax APS-C camera.
When I first got my full frame gear - after years of shooting APS-C and smaller formats - I was briefly obsessed with the ability to achieve shallow(er) depth of field at wider fields of view. I was using a 24-70 f/2.8, and pretty much kept the darned thing wide open for the first few weeks, marvelling at that sense of space and separation in my images
But here's the thing - after a short while, that novelty wore off. Shallow depth of field is fun and can be creatively useful, no doubt, but it's not the "be all and end all" of photography and is over-used, IMHO. So, in time, I found that I'd started to shoot "normally" again. Now, are there still occasions where I leverage that DoF capability? Sure... but really not that many. Most of the time, the stuff I shoot on full frame with my 24-70 f/2.8 is the same as I'd produce with my 17-50 f/2.8 on APS-C.
Which brings me to my recommendations...
If you have a full frame itch that you simply want to scratch, and if you can get away with using the EVF, the old A7 is still a decent little camera - showing its age now, for sure, and the lack of image stabilisation might be limiting... but plenty of folks still get excellent results from it. The A7 MkII, if you can stretch to it, is considerably better in numerous respects - EVF, image stabilisation, controls, lossless raw, etc. Either one, though, will get the job done - and if you use it for six months or so and decide you no longer wish to continue down that road, or you want to upgrade to something better instead, you could probably get back close to what you paid. It's a low risk option if you choose wisely and pay at or below typical market price. Do remember, though, you'll find yourself buying adapters, accessories, additional batteries, etc. - all for a different system than your Pentax gear. The cost can mount up pretty fast.
All that said...
Based on my own experiences, unless you have a specific
need for full frame (as I did and do for my Soviet lens collection), I truly think you'd be better off investing your money in additional glass for your APS-C platform. The DA21 you mention is a very fine lens - one of my favourites, actually. For that matter, any of the DA Limited primes is a joy. And the FA Limiteds... well, I'm not fortunate enough to own any, but it's quite clear that they're all fabulous for different reasons, and any one of them could provide a real creative boost