The use "that is it" and "that is all" in context

Question

Tell me please if there is any difference between that is it and that is all in the following sentence.

I have only done half of my homework, that is all.

I have only done half of my homework, that is it.

Answer 1

1: I've only done half my homework, that's all.

...is a "relatively credible" utterance, in that a native speaker might say this with the meaning you intend. But in practice few people would include the (completely redundant, imho) final clause - not least because it clashes with the meaning normally conveyed by [statement], that's all.

To illustrate that point, a context in which #1 would be perfectly natural for a native speaker might be a reply to your mother saying Your teacher just phoned and asked me to go and see her to discuss your performance at school. What's going on? (even better, What's this all about?).

[statement], that's all
means
[statement] is true, but it's not important.

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Correspondingly, the "tag" clause That's it has its own idiomatically established implications, so here's a slightly different context where a native speaker might reasonably come out with...

2: I've only done half my homework. That's it!.

Suppose you overheard your mother on the phone (talking the teacher, as above), and you asked yourself: I wonder why my teacher called my mother? Then you suddenly thought of a likely explanation...

[statement], that's it. (more commonly That's it! [statement]
means
[statement] is the answer / solution / explanation I was looking for

Answer 2

"I have only made half of my homework, that is all."

I don't want to tell anything else about this situation. I think you can understand me without more words.

"I have only made half of my homework, that is it."

This is a fact. This have some reasons, but I don't know what can I say else.

This is a difference. As far as I can understand these sentences.