This recently came up on the Dorico FB group and I couldn’t figure out a global solution there either. Do you have the Notation Express templates? Do they automatically fix it with an included doricolib file? It looks good in their preview pic, although they may have manually adjusted that in Engrave:
I think the kerning pairs are set in the chordSymbolComponentKerningPairs.xml file. Obviously there’s no setting for PetaChords. I’m not sure if it’s possible to create entries for the PetaChord comp.bass.slashdiagonal IDs with a doricolib file or not, but that’s the only idea I have, as there’s no global setting to adjust this poor spacing. It’s possible this spacing is simply hardwired in and there’s just no way to adjust it globally at all. Obviously you can create overrides in Project Default Appearances, but that’s hours and hours of work just to account for the most common possibilities.
I know @pianoleo was involved in creating these templates, so perhaps he’ll know if the Am/C symbol from the image above was manually edited or if they figured a way to fix the spacing globally for PetaChords.