I had this thinking too on the statute of limitations.
However, New York State law has some ways around it. If the suspect was otherwise unattainable, whereabouts unknown, or was simply outside the state for any length of time can pause the statute of limitations. It is likely this is what the DA will argue that due to the fact he was in unindictable as President, they couldn’t bring charges against him or did not have the jurisdiction to do so at the time.
Now that Trump is no longer President, it has only been a total of 4 years, not 7.
Keep in mind, it is very likely that Trump will indeed argue the statue of limitations and even the most liberal of judges could concede this point.