Again outside of your specifications but if you feel the bokeh of the 75 is unacceptable there is always the 55-110 zoom. However I have no idea if this has a better bokeh. These are just thoughts

I do not want to direct you away from this forum which in terms of range of knowledge probably is better than any other but as your question is quite specialised you might want to ask it on pentaxforums which specialises in things Pentax and even has a section for P645 and P67

pentaxuser