Maybe something more happened between Nemesis and Picard beyond the supernova.
The Romulans were instrumental to the victory over the Dominion, to the point where the Federation was desperate to bring them into the war.
Then at the end of Nemesis Riker was going back to Romulus on diplomatic grounds as they "wanted to talk".
Now suddenly 20 years later you had systems threatening to leave the Federation if they saved the Romulans from destruction?
There's a few inconsistencies here.
Not at all. History is full of examples of times where enemies will join forces to take out another player if its to their advantage. Those players often then would immediately revert into enemies, or at least not allies.
And we know the next contact the Federation has with the Romulan Empire is a planned attack to destroy whole Federation worlds. As for Riker going to talks, those talks could be anything. They might be we are closing our boarders until we can get our government back running, it could be the military now saying we are the government you need to deal with and that the senate is gone in both name and reality. That could literally be anything.
A fairly timely comparison is back to the original parallel the Romulans served which was to the Soviet Union. Almost every one who was a diplomat, in the State Department, or in elected office knew that the Soviet Union was not our friend or real ally. They knew before the end of the war, that the Soviet Union would be trying to take advantage of the chaos after the war ended to expand their influence. But we needed them, and they needed to us against a common threat. It took over 40 years for our relationship to start to warm up to any real degree to the Soviet Union.
In this fictional universe, the Federation decides to help. They divert resources to build a massive relocation fleet.
To give an idea of how big this is. We know that during the biggest security threat the Federation has ever faced that Starfleet had divided its forces into at least 10 fleets. That at least one of those fleets was over 600 ships (we don't know if that count included the warp capable fighters that were part of it). An we know those ten fleets covered both front line battles but also operated deep in the heart of the Federation. You could reasonably argue that Starfleet might have in total around 10,000 vessels that have some combat capacity. We also know that Starfleet could not build ships as fast as it was losing them during that time frame.
That means that building a fleet of ten thousands vessels isn't something that can be done in short order with all the facilities in the Federation.
Once that rescue armada and the ship yards are destroyed, that means the Federation is left with 3 real options.
Those options are.
1. Diverting almost the entirety of Starfleet's ships to serve as the rescue armada. As these ships aren't designed to carry a ton of passengers even in emergency's this means each ship would be making many , many trips just to handle the immediate evacuation of Romulus and Remus.
This has critical implications for overall Federation safety. There would be almost no forces left to safeguard the Federation as a whole from natural events that impact the Federation (say things like Asteroids about to wipe out a planet), piracy, and most importantly any external security threat to the Federation. With the vast bulk of Starfleet engaged elsewhere away from both inner worlds and from it's vast interstellar boarder the Federation would be exceptionally vulnerable. To a degree that even a small force could do severe damage to member worlds, outposts, and stations.
2. The Federation decides to use any private or Government owned ships as the primary means of transport. Things like ships that normally transport people from worlds to world, freighters that typically deliver supplies world to world, ect.
This also has several issues that would impact the Federation. Using what I would describe as mass transit, would basically put a serious moratorium on off world travel. Using freighters, would mean Federation citizens would have to deal with the interruption and massive delays of various supplies that travel between worlds. Is the average Federation citizens comfortable with giving up so much of what they have most likely taken for granted, for the sake of a race that has for it's history almost exclusively been an adversary.
3. Then you have the Dunkirk model, where its a hodge podge, you take any type of ship, you rush to build what you can, you use security forces, commercial and government properties to do what you can.
That options holds is what most likely would have happened, and while it keeps the at least some ships on the front line and doesn't divert all passenger or freight lines, its still a serious issue, for both security and creature comforts.
And that's just for the period of time it would take to relocate 900,000,000 (which would be months most likely at best without using vessels specifically designed for the tasks).
But then we get an even bigger issue (or at least we should), a super nova impacts notably that star system but also its radiation spreads outworld making star systems around it uninhabitable. There are words both in Federation and Romulan space that would need to be evacuated, many of them. That means this would most likely be impacting Federation security and Federation comforts for years.
I could certainly understand certain member worlds not willing to put up with that using that leverage against any relief effect that would seriously impact them.