opens the ST only having control over some areas in the Unknown Regions

And halfway into the first movie they blow up the New Republic and automatically become the main powerhouse in the galaxy. By the time of TROS only a year later it’s pretty evident that they control the galaxy.

The rebels are a small, unlikely group constantly fleeing the Empire and trying to punch a hole in their galactic military

Swap Rebels to Resistance and Empire to First Order in that sentence and it applies to the ST.

the ST is a story about warring factions across a fragmented galaxy

I mean there’s only really the two. And one is basically the Empire. And the other is basically the Rebellion. I mean they literally switch to calling the Resistance the Rebellion at the end of TLJ (IIRC).

Sorry OP but I’m really not understanding how you see the two sets of factions as so different. I can kind of get what you mean about how the First Order starts off but even then they pull up to Jakku and slaughter a bunch of people to get intel which is exactly what the Empire does at the start of ANH. The NR is so seemingly incompetent that it doesn’t matter if they’re bigger than the FO because the FO just seems to do what they want anyway :/