Hello Julie, welcome to the Forum!
It sounds like you have a great starter kit, I'd suggest using it a bit and checking to see what zoom-setting (focal Length, FL) you use most. Walking around, urban and scenic would be in the 20mm-35mm range, the extremes or 'ends' are for much wider or closer framing. Everyone seems to develop a favored FL (or two) of seeing and composing a scene. Zooms are a perfect way to find it or narrow the range down.
If it's anywhere near 35mm, the DA 35mm f/2.4 would be my recommendation. There's a thread called 'Plastic Fantastic' that will provide plenty of examples for you to view. Started by todd, it's here;
https://www.pentaxforums.com/forums/122-lens-clubs/196639-da35-f2-4-plastic-fantastic-club.html
With the DA 50 and DA 35, you can begin trying lower-light photography - indoors, evening, stage or performance, plus subjects with movement can be done with a slightly faster shutter speed. They also provide smoother Bokeh, aka background blurring, giving you better subject isolation. This is especially useful for portraits and people shots.
The downside is you must zoom with your feet, moving to provide the various views a zoom lens would normally give you.
Last, you're in luck with any prime lens selection. Since Pentax has kept the K-mount for over 30 years now, and older M42 lenses work with a simple adaptor, literally millions of used K-mounts are still around.
Hope this helps, good luck and post some photos!
Ron