I'm not even talking about Grievous here. Honestly, I think that the Yuuzhan Vong would have made for better antagonists than the Nihil because the High Republic doesn’t have a centralized military at this point, mostly just independent system defense forces for each planet. The Yuuzhan Vong would give the High Republic a rude awakening to get its crap together and actually unite to fight back against the Vong or face extermination.
the lack of a centralized military is exactly why the yuuzhan vong do not work for the High Republic era.
the HR is not a story about a complacent galaxy needing to be violently “woken up” by an extinction level invasion.
It is about a young, optimistic rrepublic learning, often painfully, that good intentions, decentralized power, and moral authority are not always enough. the conflicts are meant to test ideals, . dropping the vong into that setting skips the entire philosophical arc and jumps straight to an end of era crisis.
the Nihil in turn create conflicts that cannot be solved by brute force alone. they force characters and institutions to confront abstract problems like trust, coordination, authority, and moral compromise,the vong reduce most conflicts to survival math. unite or die. in turn the nihil are a distorted reflection of the republic itself, decentralized, idealistic in their own way, and empowered by freedom without responsibility. they emerge from within the same system the republic believes in.
more importantly, something like the vong wouldnnt really make sense for the time period the HR is set in. Its clearly meant to evoke Star Wars’ fantasy roots,the Knights of the round table, a mythic “golden age,” and an era of optimism and exploration. the vong are more like endgame bosses,they function almost entirely as an invasion story, designed for a setting where the galaxy is already exhausted and fractured.
honestly, the vong haven’t aged all that well either. A lot of their writing relies on trying to appear mature through excessive sadism, masochism, and shock value,which feels pretty dated now. on top of that, their religious symbiosis with the wider SW universe is kind of undercooked. later HR stuff especially Phase II, with factions like the Path of the Open Hand does a significantly better job exploring religious cults, belief systems, and their relationship with the force in a more thoughtful way.
The vong would fit better in either the old republic or long after the sequels.
Also i'm not sure if your up to date with the HR or not but the Nihil arn't really the main villains of the High Republic either