Hi! I have more information about と adverbs. Apparently, there are 3 kinds of と adverbs:
- taru-adjectives: these take と in adverb form and take とする when they are used as adjectives in modern texts. After learning this information I’ve encountered many taru-adjectives in the wild.
- Some of the na adjectives take と in adverb form.
- Onomatopoeia.
*In the last two options, there are cases と is omitted.