This

Trump can pardon anyone for all actual or potential federal offenses, whether before or after they’re charged. It’s what happened with Nixon.

State charges are a different matter..... the real question is whether DC offenses are federal or state.. IMO, that issue will probably go to the Supreme Court next year.

Isn’t DC always federal?

It’s a hybrid. Among other things, it has a mayor and a city council, but is also considered a federal district. So, it’s not clear what local laws (which everywhere else would be state laws) would be treated like for constitutional pardon power purposes.

There are all kinds of weird constitutional issues about DC. It’s why the Heller case (the most important case about the 2nd amendment) was brought there.