If you can get close enough, why not?

It used to be like that back in the 70s & 80s. Quite a few makers used the K mount besides Pentax; Chinon, Ricoh (almost - that dreaded pin) and Practica come to mind at this late hour, so it was quite feasible to have compatible bodies and lenses with quite a variation of badges on them. I nearly bought a Ricoh but the dealer couldn't get the metering to work.

As to the mirrorless EVF's, its not the same as SLR TTL so horses for courses. My experience of EVFs so far has not been favourable though no doubt they are improving.

That was just one example, I suspect it breaks down when one considers accessories such as ring flashes and other gizmos. I'm not sure, though, how one gets proper TTL viewing with a mirrorless camera though? I can't see that will work as at best it has to be a laggy VDU.
You'll appreciate I'm not convinced with that suggestion! The only obvious mirrorless K-1 FF alternatives I can think of are the Sony's. I could have got one of those quite some time before the K-1 came out if I had thought that was a sensible idea. I did consider it... and discounted it!

IMHO, brand loyalty revolves around being able to upgrade part of a system rather than having to dump everything and start again, be it a new camera body or better lens. (I realise there are some who seem to be prepared change everything quite regularly though!) Pentax are probably favourites for brand loyalty given their long history of lenses and those of third parties that are still compatible with the newest camera bodies without requiring any modification to anything other than setting a menu option or two. And many actually like to mate ancient lenses with the newest bodies.

But does it come with a free reflector?

Oddly, I saved one of these for this purpose quite some time ago.

Thanks, I can see that now. I was setting HS but it wasn't displaying it.

Ah, so Pentax's are cleverer than that other brand beginning with N and don't need it. I'll try that. Thanks.
I see the HS only kicks in when you go above the camera's sync speed. That explains why I wasn't seeing HS on the flash display even though it was set. I'd have thought if you'd set it it would display it but not bother using it until it became necessary. Confusingly implemented on my 360 if I may so so.

Which menu option do you set on the camera to allow HSS? I've watched a video about using this on another brand beginning with N which shows a star against the sync speed in the camera's menu option when HSS is achievable with a compatible flash gun but cannot find anything similar on any of my Pentax's.
I'd like to try it but I'm failing at the first hurdle!

The DA15 isn't any real use as a full frame lens.

Seems SRS do have the Samyang, though very slightly dearer than the link I gave earlier.

I got mine from Amazðn link. It's gone a a few ££ since but still great value for what it is. Current price is £284. Don't think SRS do these.