Some random thoughts:

My parent always had a number of narcissistic traits, but was never a full-blown narcissist.

Narcissism is one of the "Cluster B" personality disorders. Other disorders have some similarities and some differences. (I don't even know if they are clearly separated in the territory. The psychological definitions keep changing.) So if there seems to be a serious issue, but it only partially fits the definition of narcissism, and partially it doesn't, maybe it is something else. My quick uneducated guess would be borderline personality disorder.

If you are not sure what to do, maybe it would be better to not burn the bridges, only build clear fences. (You can still burn the bridges later if you decide to, but maybe with good fences it will not be necessary.) For example, move to your own place, and visit your parent once in a month, for an hour or two. Also decide which parts of your life you don't want to share with them, and simply don't talk about it. Maybe find a topic that is not painful for you, and always switch to that one (e.g. talk about work, or news, or latest science; don't talk about your health and relationships). Maybe this will work.

Complicating factor

Uhm, I think your decision should not be influenced by this. Solve your personal problems first, optimize the world later. (Having your personal problems unsolved will also harm your world-optimizing abilities.)