[RPG] How does Overchannel work with Cantrips

Yes, that is what the text is saying. Bigby's hand is a spell and can deal damage. If it does deal damage, and you've overchanneled, it deals maximum damage. The number of separate rolls involved and the time between casting and damage and all the other things like that are completely irrelevant to the rules in both descriptions.

The only reason this seems unclear, as far as I can tell, is because overchanneling seems more better for this spell than others, so people are hesitant to let it apply. In actuality, overchannel nearly always simply doubles the expected random damage of a spell; it's just that this spell is capable of a lot of total damage so the absolute expected gain is larger.

Other benefits of overchanneling, like a deviation of 0, apply equally to this and other spells.

No, in this case you need to decide whether you're going to use Overchannel when you cast the spell.

The timing for using Overchannel is "when you cast", and a spell you can use it on is "a wizard spell of 5th level or lower that deals damage". This is merely meant to specify that you can only use it on spells that include damage as part of their effects (like fireball), as opposed to spells that don't deal any damage (like invisibility).

If it was meant to be decided after it had dealt damage, it would have to use wording like "When you cast a wizard spell of 5th level or lower and deal damage".