It is difficult to give you the best advice without more details on your existing lens(es).
If you have the kit lens(es), I assume that you are after quality lens(es). Then the following my apply.
The FA31mm f1.8 is definitely a great lens. The IQ is outstanding. The focal length is almost ideal (not too long, not too short). The lens is fast. I got mine new from B&H during a special (for less than US$1,000).
A few more suggestions:
-If you are interested in portrait, you could consider some lens between 50 and 100 mm.
-Are you prepared for the challenge of MF lens? See
https://www.pentaxforums.com/forums/pentax-beginners-corner-q/146443-manual-f...we-afraid.html
-Have you considered second hand lenses? There are ample choices incl. quality lenses at the PF marketplace.
From my own experience, and in addition to the FA31mm, my second recommendation would be my 2nd favorite prime: Voigtlander Nokton 58mm f1.4. A fast prime with superb IQ, MF, very solid.
Food for thoughts...