What if the sequels took place after a Cold War type stalemate? The remaining Empire on the fringes and Republic exist with a DMZ boarder.
The fleet at the end is the fleet that always existed.
Starkiller base isn’t a new super weapon in itself and is never used. It breaks the peace treaty by manufacturing death star weaponry to be equipped on their fleet.
Leia’s group isn’t the only force in the galaxy, just the most forward operating group that is allowed by treaty.
I’ve suggested something similar to this. Basically, the New Republic and the First Order have a truce, and most of the action in the trilogy takes place in First Order space.
The First Order in TFA is focused on finding Luke, which doesn’t violate their treaty, but Leia is against this. So, since the Republic can’t take action, she forms a Resistance movement within First Order territory.
Starkiller Base is not a super weapon, and no planet gets destroyed. Then, in TLJ, the Republic can’t help Leia and the Resistance because it is outside of their jurisdiction. First Order territory, First Order rules.
I actually think that you could make it to where the First Order doesn’t really violate the treaty until IX, when it seems like the First Order are going to try and invade the galaxy using their newfound fleet.
It could be cool if you could imply that the First Order attacks the Republic and there is a war going on off-screen, but I feel like you would need new scenes to really make that work.
The pro is that the Republic isn’t destroyed. The con is that the stakes might not be as high since First Order hasn’t taken over the entire galaxy. But since that “war” is never really shown on-screen, you could argue there may as well not even be a galactic scale war.