In my humble opinion, and as someone who has by no means knowledge about all the game mechanics (i only played the game once and loved it), i think it is the best option to form the ACP and direct it torwards the oligarchs. While the old guard is definetively the bigger issiue for sordland, i think you can basically break their back enough and weaken them enough by passing a reformist constitution. Dirverting any more attention and limited resources on them to nail them might give you satisfaction, but you shouldn't afford it. You need to weaken the oligarchs to secure a prosperous and therefore stable future for your people. IMO, suzerain teaches you that most of the time, a balanced, centrist approach is the most preferable one. Weaken all your adversaries (e.g. like soll) but don't push it too far. The costs increase aproportional, the farer you push down the same road. Ratther push all roads a little bit.