Anki review iss are not scheduled, and could best best done during your commute.

You would only need a comma here if you repeated the subject, making two independent clauses. Example: Anki reviews are not scheduled, and they could best be done during your commute.

A more detailed explanation of this, targeted at native English writers, is here:
https://www.grammarly.com/blog/comma-before-and/
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I switched it to the plural 'Anki reviews' because this allows it to be treated like a mass noun, so you don't need any determiner before it. You could also say, "The Anki review is not scheduled and could best be done during commute", but this refers to one specific review, not all Anki reviews as a general group.
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'could best be done' and 'could be best done' CAN both be grammatically correct. However, they have slightly different meanings and 'could best be done' has a more general meaning, so it's used much more frequently.

'could best be done' = this is the time that is best to do the reviews (for any reason)
'could be best done' = this is the time that is best to do the reviews because the reviews will be completed with the highest success

I find it useful to imagine where I would put 'not' in the sentence, although I don't know if that helps when English isn't your native language. "could not be done" is a lot more common and less weirdly specific in meaning than "could be not done".

Example:
My Anki reviews need internet to provide me with text-to-speech pronunciations, so they can not be done during my commute.
(good)
My Anki reviews need internet to provide me with text-to-speech pronunciations, so they can be not done during my commute.
(bad - sounds weird and technically means something different)
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'commute' needs a determiner. If you don't want to use one, you can say 'while commuting' instead:

Anki reviews are not scheduled and could best be done while commuting.

GA grace period of five minutes means that someone is allowed to be late for five minutes late.

Like 'Anki review', 'grace period' needs to be made plural in order to be treated like a mass noun: "Grace periods of five minutes mean that someone is allowed to be five minutes late."

You can also use a determiner with it, as I did in the correction. To me, this sounds a little better in this case, because you're referring to a specific kind or instance of grace period. Reading the Wikipedia page on grace periods gives lots of examples of when to use 'a grace period' and when to use 'grace periods':
https://en.wikipedia.org/wiki/Grace_period
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'late' doesn't need 'for' to talk about how late you were. That can go right before it, just like quantifying adverbs used with other adjectives:
The tree is 30 feet tall.
She is ten years old.
He was ten minutes late.

It is the best in the state of New Jersey, if not in the United States.

Adding 'entire' is a very common in this kind of construction: It is the best in the state of New Jersey, if not the entire United States.

It may be the best, although in the United States, although I cannot guarantee that.¶
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It may be the best in New Jersey, but I cannot guarantee that it is the best in the United States.

Instead of New Jersey, you could put any locality in the US. "It may be the best on the East Coast, but I cannot guarantee that it is the best in the United States."