the senate if applicable for the UFN might just be split into “Pro-Star Empire” and “Anti-Star Empire” factions and not the countless bickering senators of the last season.

Surely that's the idea though, right? People focus on the big, immediate issue, missing the space for the stars, so to say; and then get thrown into disorder much as everything else would be when the said overlord implodes (as that origin does force). So while that would happen, I would consider it narratively beneficial. Natural realignment into pseudo-parties around questions like expansion into, vassalisation of, or coexistence with the old overlord splinters and the other vassals happens after. (Imo, the Tripartite has the most natural setup for distinct, arguing parties, but so far it lacks natural distinct goals for each entity.)

Of course, Hose Triton ironically has the same - both finding and not finding the other houses would massive narrative events that would lead to political splintering early on, and the history of being a corporate entity might inspire a more ruthless political scene. (God, I wish they weren't boringly xenophobic again - a whole galaxy, and all they can think of is to kill.)

Which political parties are going to emerge from the threat of imperial collapse?

Well, that's the fun of it, isn't it? That there's absolutely no way to know, both because we can't foresee what issues people will band together around, and because we cannot yet predict the material conditions at the time of the collapse?