Learn English – Do people commonly say “go down” to mean “go to sleep”

It's not common to use "after a long interval" to mean "after a long time." Your sentence "I had sushi for the first time in a long time" sounds perfectly natural.

Yes - You say that, but [some refutation or caveat] is idiomatically commonplace for many native speakers, in response to an assertion they disagree with.

It's nearly always used in a context where although the speaker doesn't fully agree with the preceding assertion, he recognizes that there's some truth in it.

In fact, you'll often hear You could say that, but..., which more explicitly acknowledges that the assertion is at least "credible", even if it's not completely accurate. Note that in this usage, you = one, anybody, not necessarily the person being addressed (i.e. - what it amounts to is something like It wouldn't be ridiculous for you/someone to say that, but it's not entirely true).

Also note that if the personal pronoun is stressed (You say that, but...), the implied meaning shifts significantly. This form almost always occurs in contexts where the speaker completely disagrees; he intends to dismiss your opinion on the grounds that you're ignorant or biased, and/or to cite other sources more authoritative than you to refute what you said.