The DeSantis-DoJ kerfuffle is less than it seems

The letter does not actually state an intent to bar federal officials. It also does not contend that, if DOJ articulated a basis for federal concern, the state would still object. The federal officials could simply show up and force Florida to seek an injunction to remove them. The federal officials would have the edge in such a court challenge.

The Trump Justice Department sent monitors to some polling places in 2020. However, they agreed to remain outside while monitoring conditions. Florida was objecting to suggestion that the Biden Justice Department intended to station officers inside of the polling places.

There is some indication that the Biden Justice Department follow the same approach under the Trump Administration and remain outside. However, federal officials could seek entry if they see or hear of a position inside of the polling place. I would be surprised if the state would seek to bar entry in such a case given the concurrent federal jurisdiction in elections.