"That's it" versus "That's that"
Solution 1:
I think you're right in that there's a little difference in nuance, but not a lot.
That's that to me means End of Discussion.
That's it to me means that's all. I do believe there is more flexibility.
You can have either grape or orange juice to drink, and that's all.
You're grounded and that's that.
I kind of like, "That'll do."