in all of the numbered folders mentioned in the tutorial, there's no amethyst armor trims, so i do not know how to format adding it to a different light level.

Armor trimmed with amethyst is handled a bit differently, since there are no additional item IDs for it. The property is checked directly via predicates in /data/dynamiclights/predicate/entity/light_level/6.json. The responsible code block is

{
  "condition": "minecraft:inverted",
  "term": {
    "condition": "minecraft:reference",
    "name": "dynamiclights:entity/has_amethyst_trimmed_items"
  }
},

To change the light level, move this block to the desired <light_level>.json file.

Note that, similarly to tag/tag folder names, there is also a predicates folder (with an "s" at the end) for Minecraft versions 1.20.4 and below. For versions 1.20.5 and 1.20.6 only, this file is overridden by the one located in /overlay_35/data/dynamiclights/predicates/entity/light_level/6.json.
Apply the change to those files as well if you want it to apply to these versions.