The use of economic propaganda to bring down a galactic empire sounds a lot like the LaNague Confederation series by F. Paul Wilson. In particular book 1 from 1980 , Enemy of the State.
In Wilson's novel, he extends the "squeeze" of confiscatory taxation and currency debauchment to a conclusion involving a Weimar Republic-style hyperinflation that brings down a galactic empire – and from which humanity's only hope of rescue arrives in the form of an anarchist conspiracy to complete the Empire's downfall and replace that government's "official counterfeit" with honest money.
Other points that match:
One method used is leaflets
Check. The Anarchist group prints out regular leaflets advising folks about the pending downfall of the government/empire. The leaflets look like this (image from the book):
There is a single planet that is not part of the Empire. Due to political instability in the Empire, a new enemy is needed to focus people and the sole planet is designated as the enemy.
Partial check. There are actually a handful of planets not associated/part of the empire including Tolive which is the planet from which Peter Lanague the agent comes as well as Flint, a 'warrior culture' planet. But, the empire does use scapegoating of other worlds such as Tolive (and eventually
an alien species which is discovered.
A single enemy agent.
Check. Peter LaNague is the primary agent from Tolive who is sent to bring down the empire. Though he has help from two "Flinters" (think mystic ninja warriors crossed with economic gurus :-)
I don't remember the part about the public floggings. But, it has been quite a while since I have read the series. Too, the Empire is NOT based from Earth. Instead Emperor Metep VII rules from the planet Throne.
Description of the Imperium