@ff2000 - thanks!
Checking equery and portage, it seems that installing kde-cli-tools requires installing 36 other kde packages, probably bringing in the plasma desktop. Our download bandwidth is limited - adding so many new packages just to be able to use dolphin instead of spacefm to manage file defaults seems not the best course.
Hopefully file association changes made with spacefm will be picked up by dolphin. Otherwise I’ll probably just remove dolphin from my computer.
I’m still guessing there is a file somewhere that can be modified directly to manage file associations, if only one knew “which file”.