I think what you want to do is to add a load point to load json files from the plugin and here you want a priority of > 10 to allow the acf-json folder in the theme to be loaded first. This will mean that the JSON file from the plugin will only be loaded if that JSON file does not exist in the theme. Create an acf-json folder in the theme folder. When you sync and then save the field group it will save it to the theme folder and override the json file in the plugin. But doing this will also mean that if you ever updated this field group in the plugin that those changes till not effect any site where they have been changed.
This is perfect and points me in the right direction, thank you so much!
