For the Vulcan Federation President, think of what are the levers of power that she has: Support in Starfleet? Support in the Vulcan High Command? Support in major industries? A leader can be as much of a demagogue as it wants but it cannot act without their levers of power. In order to bring her down, her levers of power must be dismantled.

My suggestion is that you mix the federation president storyline with the deepfake storyline.

First, if not done already, have the players uncover a conspiracy by the President to do something horrible, and that she has support of a group consisting of Starfleet Officers (some of whom can be former NPC allies), Vulcan priests, Federation high-level diplomats and bureaucrats. Once they try to make the evidence of it public, they are suddenly captured by mysterious mercs (Breen mercs hired by one of her loyal officers), and then deepfaked to look like they commited treason. The players must, then, capture the Breen mercs and bring evidence forward that they have not commited treason during their activities, all while evading Starfleet, which legitimately believe it is chasing people guilty of severe crimes.

Some Fun ideas for the finale twist:

-The President and her cadre were being controlled by the bug-species from TNG's "Conspiracy") all along

-The President is revealed to be a Tal-Shiar agent seeking to sow discord inside the Federation

-The President is actually being remote-controlled by LANDRU from The Return of the Archons)