Garlemald is not Archades.
The Senate was an advisory committee split between the Optimates and the Populares who didn't exactly work with each other so I don't understand how or why they could collectively do evil together. Especially since only the Optimates seemed to support the emperor. Then there were the Populares, who already existed in the game, and were written to be sympathetic by being against the Empire's conquests but went through the proper channels and just suggested reforms until they began to be purged by Varis and their supporters got gassed. Besides, I don't know how you consider them to be "Sharlayan style democracy" when Garlemald was a republic for ~600 years with no Sharlayan influence at all until Ascians got a hold of them and made them an empire. Landis was another republic in Ilsabard that had no Sharlayan influence.
If you want your "evil" force that assassinates nobles and kills civilians, look no further than the Lemures in the Reaper storyline who tried to assassinate Solus for trying to conquer lands that had nothing to do with the original Garlean goal. Or Eula who killed her dad and some others and was supported by Solus because of it. Or Titus, Varis, Nerva, and Zenos who mobilized their legions to fight for the throne and destroyed Garlemald over it. The scheming nobles playing their game of thrones and the actual assassination force that exists in-game are more likely culprits to be doing dirty deeds than a group of elected civil servants. The Senate was already established in the game by the time of Stormblood to be a potentially moderating force so unless you want to re-characterize Maxima and the Populares, it's just plain odd to do what you did.
It doesn't feel like your rewrites would fix anything. It just feels like FFXII randomly stapled into the FFXIV MSQ at best, and fanfiction that ignores existing lore in order to satisfy your preferences at worst.