Is it possible to make a config that lets you set that all diamond gear needs less resources? Or do i have to put each different diamond gear in the "overrides"?

You would either have to add an override entry for each item or use a tag, examples:

items: ["minecraft:diamond_axe=minecraft:[email protected]", "minecraft:diamond_pickaxe=minecraft:[email protected]"]
tag: ["tag:my_datapack:my_tag=minecraft:[email protected]"]