Latest possible HRE Imperial Reform

With a POD no earlier than the Protestant Reformation, what's the latest possible date for successful Imperial reforms that would strengthen the Empire, moderately centralize power, and ensure the Empire's longterm survival?
 
December 2, 1805. If the coalition had won decisively at Austerlitz, scattering the French army and perhaps capturing or killing Napoleon, in such as way that much of the glory of the victory falls on Emperor Francis II, there would be a window of opportunity for Francis to turn his political and diplomatic capital into support for moderate but significant imperial reforms aimed at establishing an effective counterweight against future French expansionism.
 
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