TBH the romulan government being so pig headed and arrogant and its leadership filled with assholes who would rather pretend nothing was wrong then admit weakness enough to assume they could contain/delay the supernova indefinitely is fairly in character for them, and would not only tie in nicely with how the klingon government had its own chernobyl in movie VI but also correlates nicely with china's failure to deal with the bat-soup-shits these past few months
Hell if you really wanted to use established canon to cobble together an explanation as to why the supernova was so fucking nasty and galaxy threatening without delving into convoluted conspiracy and iconian fuckery, have it be revealed that the romulans, in the ultimate gesture of arrogance
were using omega particles to fuel the containment shit around/within it, thus ensuring that when disaster inevitably struck, it did so in a way that would utterly annihilate subspace on a galactic level and thus end interstellar transport forever, which spock and friends only found out at the last minute and thus necessitating that whole red matter horseshit as a technobabble way to reverse its effects/prevent it spreading further than the Romulus system, but even then it seriously fucks up just about everyone in the alpha quadrant in the same way a massive EMP pulse would fuck up earth, leading to widespread death and suffering and starfleet paralysed until the red matter shit at the heart of the omega explosion is able to repair most of the gaps in subspace.
Once this happened, and the federation higher ups found out about how the romulans almost destroyed all interstellar civilisation because their government was so fucking selfish and uncaring that they would rather risk galactic annihilation than suffer the inconvenience and minor political embarrassment of having to move their capital world because of standard cosmic phenomena, despite the federation bailing them out with shinzon, and after centuries of selfish atrocities and betrayals and duplicity aimed at them and their allies, starfleet command and the higher echelons of government finally snap and decide that they are an existential threat that must be subdued, even though they are already a broken power and complete non-threat to anybody at this point.
Have this be the starting point for a federation that has been so jaded by borg incursions, war with the dominion, and now this shit pulled by their oldest enemies, coupled with the fact they are now the sole superpower in the alpha quadrant because of all the aforementioned events hitting their rivals and klingon allies far harder, that they are actually experiencing a widespread and distinct crisis of identity after so much shit in the past few decades, and that this could go in a really nasty direction if things dont improve as ordinary citizens have not only had to deal with war and fear of assimilation for a generation now, but also had a near apocalypse caused by a supposed ally who had already been screwing them over their entire history.
I know it all sounds fanficky as fuck but atleast its not "THUH FEDUHRAYTION IS NOW RAYSIST BECAUSE TRUMP AND BREXIT!" for gods sake