Going back to S1 would allow for some exposition from Picard regarding the evacuation (i.e. Yuyat Beta), so we can understand what went wrong and why Starfleet needed to build the synth ferries in the first place.
I don't think they need to go super heavy into it, but the explanation from the novel worked well enough I thought and could most be summed up in a line of dialouge.
It described that they needed to build alot of ships, quickly. While the lions share of the work could be just normally automated, there are several starship components that required skilled laborers to assemble. It never goes super indepth as to WHAT and WHY, just that there are components that require a skilled hand that can problem solve. They just didn't have enough people to do it, and training took much longer than they had.
So Maddox was kind of enlisted to help work on it, and while he still couldn't get his positronic brain working properly, he coat-tailed on the bio-neural gel pack technology to fill in some of the gaps.
Tl;dr, they needed a much larger skilled workforce to build enough ships to evacuate the Romulans.
There IS an inherent problem in this era of Trek of asking... why not just use holograms? My preemptive answer to that question is probably that an actual sentient, truly problem solving hologram is rare and steps have almost certainly been taken by that point to prevent it from happening, to avoid a Moriarty situation. The vast majority of Holograms just APPEAR to problem solve and such, it's all just straight programming, and they're limited by the computer core... the more holograms you have going, the more they're splitting computing power.
Of course the next question is then, why not just build more computer cores? I assume the answer creates something of a circle... they COULD just build more computer cores in order to have holograms doing all of this, but building the computer cores would take just as much, if not more, resources than building the synths.
Storywise for this hypothetical continuation, I think it would be interesting to echo Season 1 to some extent and have Picard basically having to go rogue again, and alot of people siding with the non-Picard side. Here's my idea:
The Jurati Borg know what the rift is. They didn't have time to explain, but they know. We pick up with them still standing guard, but they know they can't hold it forever. They appeal to Picard to try something, and he knows he needs to try to communicate with them.
Starfleet pretty much tells Picard... no. No way. First off, Picard is a synth now. He wants to take Soji, the synth who helped initiate the call to these aliens in the first place, and Data... who is also a "synth" and has the pattern of the scientist who helped create many of those synths... to help THE BORG open the rift to "talk to" the aliens who wanted to destroy all organic life the galaxy? Are you out of your mind, Picard?
Then yeah, the Zhat Vash get wind of this and are ready for Round 2.
I don't really know it all ends, but it has to be peaceful. The only thing i've got on the top of my head is Picard goes into their realm to act as a sort of ambassador. I have a nugget of an idea that puts a twist on things, not fully formed, but perhaps there's something there with the Borg "virus" thing that infected all the young people... while the Borg were using it to try to assimilate the Federation, it actually had some kind of unforseen benefit, and also have the Reaper Aliens have a not-so-evil motivation. EXACTLY what, i'm not sure.
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