Jan 6th IS the end.
Read that letter again, clear as day
“The January 6th vote... is the lone constitutional power remaining...”
There are indeed other Constitutional actions that the President can take after Jan 6th.
Give up all you want baclava, say ni hao (你好 ) to your masters for me. I know those people far better than you do, and I refuse to give up. I know what life is like there.
January 6 is not an express constitutional date.