In the wake of Palpatine taking supreme control of the entire republican military and government, I would establish that the new Republic made their own military forces more like a joint fleet of smaller factions as well as making the Republic a lot more like a federal state - to prevent a single person from taking over in one fell swoop. Key is "show, don't tell". Maybe a short scene with Leia negotiating at a table with several fleet commanders talking about how to deal with the threat of the First Order. The First Order should be established too. I'd suggest that the First Order should be based around a well-defended base that originally acted as an imperial foothold to more distant regions that they never quite managed (or bothered) to conquer propperly. Something with a big shipyard that acted as a rallying point for retreating imperial ships after losing their Emperor.
With that out of the way, let the movie play out pretty much the way it did from the start, but replace Starkiller Base, since it's completely ridiculous that the First Order can suddenly build such a massive super-weapon in such a short amount of time. It took the fucking Empire longer to plan and built the first Death Star. Also, it's too similar to ANH.
Instead, I would suggest a spacestation that floats above a gas giant that is protected by a sizeable fleet of Star Destroyers. The New Republic doesn't really know what's going on there, they assume the First Order is refurbishing Star Destroyers, since clearly some sort of construction is going on.
The NR decides to perform a preemptive strike and sends their joint fleet to engage the FO, leaving only token fleets at the various home planets of the NR (against Leia's wishes).
While they get into a fight they realize that the Star Destroyers behave a bit weird, the shields seem to be weaker, but they appear to be more maneuverable and quicker than usual. Also, there is no trace of any flagship or more modern Star Destroyers that the FO has built.
So, it turns out that the Star Destroyers haven't been refurbished, they had been stripped of their main reactors and their hyperdrives (making them lighter and thus more nimble but also reliant on emergency power supply) which have been put inside the space station, which begins to funnel up material from the gas giant ion order to power a super weapon.
The whole thing was a trap and the superweapon is not a giant space pew-pew laser, it's actually a weapon aimed to fuck up Hyperspace, which makes the NR ship's hyperdrives overload, preventing them from jumping for quite some time.
Meanwhile the NR fleet is crippled, the actual FO main fleet splits up and jumps to the homeworlds of several powerful groups within the NR fleet, essentially forcing them to tell their fleet to stand down (or have their homeworld get fucked up by orbital bombardement). Thus, the NR fleet is reduced dramatically in size and capability.
The protagonists should also have some sort of victory, so maybe while not managing to shut down the FO, maybe they managed to sabotage their plans enough so the FO doesn't manage to take over the NR completely. By not being able to subdue the entire leadership of the NR, the FO would be forced to keep parts of their fleet at the various homeworlds, binding a lot of their military resources in different places, spreading them out.
The NR then pulls a "
Vichy" and admits defeat, forcing Leia and some of the people still loyal to her to flee - making her group a... y'know...
Resistance.
TLJ could then start the way it did with a small fleet of Resistance ships on the run from the FO main fleet.
