If you can accurately diagnose a DEFINITE imminent threat from SPECIFIC and agreed upon mental illness I'd have no problem with that being a temporary suspension of rights. However, there can be no confiscation until it's a completely KNOWN diagnosis, not merely a fear of such a situation. Until there is absolute PROOF of incapacitation rights must remain inviolate.
You must also implement a system for rights restoration that is simple, accessible, and not an economic or emotional burden. You must also ensure the safe return of all confiscated goods, in original condition.
When we say 'fix mental health care' what we mean is begin a program to alter cultural perceptions of mental health, and guarantee absolute access to mental health care for all citizens permanently. That means every American gets access to monthly or weekly counseling, we end social stigmas regarding mental illness, we fund mental health research (NOT pharmaceutical research but theory and treatment), we implement legal safety nets to protect those needing and receiving treatment from prejudiced treatment, etc.
It also requires addressing the causes of acute mental illness...that means changing our food and diets, alleviating stress, providing economic stability, etc. It is these factors which 'drive people crazy' in the sense of many of these incidents. It's not so much mental illness in the classical sense (long term psychosis, sociopathy, chronic dissociative disorder, etc), but temporary 'insanity' brought on by social and systemic failures. In other words, they're not mentally ill, they're angry, sad, and hopeless.
It's been known for about 60 years that the reason other nations have lower crime and violence rates are the cultural, social, and systemic factors that impact these areas, as well as things like national homogeneity, political efficacy, family unit cohesion, lower wealth concentration, and so on. There is no causal relation between crimes or suicides and firearms, as proved in numerous studies. In other words, if you're worried about events like we've been seeing then the first thing you HAVE to do is stop talking about guns completely.