Hmm, I’m not sure if this is what you’re looking for, but this is what I had to do to even get a config file in the home directory in the first place.
Run fastfetch --gen-config jsonc and it will generate it in the ~/.config/fastfetch folder. From there, you can set the parameters which are all listed here:
The changes made to this generated jsonc file are persistent.
I also found some presets in /usr/share/fastfetch/presets, which was listed in the command fastfetch --list-data-paths.
Hope this helps 