1. Do you know any words with opposite meanings that use 4 out of 6 letters, suitable for making a dice? For example, risk/rich, fact/fake.
2. How would you describe this kind of words or phenomenon, "dice words"?

Hmm the only way I can think of to describe this, is “partial anagram.” But that’s more broad than what you’re referring to. Here is a site that generates anagrams, not specifically in the way you describe, but maybe it would help:

working on /everything theory: https://everything.fun

Mathematics/Design linktr.ee/sinusoidalsnail
⟢⟢ᚖ⟡⟡ᚖ⟡⟡ᚖ⟡⟡ᚖ⟡⟡ᚖ⟡⟡ᚖ⟡⟡ᚖ⟣⟣

Hmm the only way I can think of to describe this, is “partial anagram.” But that’s more broad than what you’re referring to. Here is a site that generates anagrams, not specifically in the way you describe, but maybe it would help:

https://wordsmith.org/anagram/index.html