To my mind they have just gone with a compression.
All of the story, from the construction of Barad-dûr to the sack of Eregion etc will take place during Isildur's lifetime.
I'm not sure that they'll take the Third Age action very far past the Gladden Fields. As you say, the Harfoots heading West might be a good way to draw a line under everything.
Canonically, of course, the Harfoots did not cross the Misty Mountains into Eregion until sometime after Third Age 1050 when the Shadow took possession of Dol Guldur. But
The Rings of Power displays little respect for the canon of Tolkien's legendarium.
