Are you using the DA50 in APS-C or FF mode? In APS-C mode it would match the field of view of the FA77 (I assume you have an FF camera), in FF-mode the DA50 would vignette badly at the edges. I think I would start by selling off that one.
Now for the more difficult question of what to replace it with. I would consider both the FA43 and the quite new FA35.
HD Pentax-FA 35mm F2 Reviews - FA Prime Lenses - Pentax Lens Reviews & Lens Database Both are good and very compact. I often prefer that field of view for landscape and street photo.
The DFA 24-70/2,8 are quite heavy and expensive, not my cup of tea, so I have a hard time recommending that, but you may have a different view on weight and pricing. For the ease of zooming I would stick to the DFA 28-105. Its lighter, cheaper and has more ease of zooming. I consider the 24-70 a kind of akward compromize between zoom and prime, having all the negative sides in one, and being medicore on both ease of zooming and aperture. Yes, I expect a hailstorm of comments on that view.