I think it would give them insight but regardless it likely won't reduce their pain until perhaps later, as loss is still loss and will need to be processed. They will potentially be acutely aware of your absence especially when it comes to familiar patterns of before. Your rationalisation likely wont mean much vs emotion and the fact you are simply gone and don't occupy the space you should, yet the world keeps turning like a gross insult of your memory. For me, certain films take me back to that and that absence and self reflection intensifies. Until it passes but I don't watch those films any more because he is not there to sardonically pull them apart for their absurdity, especially when it comes to jumping through a plate glass window and not needing A&E straight after.
On some level, they will still likely feel like they could have made things better somehow or tried harder to do something. Even if you eloquently write how that is not possible they will likely ruminate on the shoulds and oughts long after you are gone. If you tell them you love them they may well still feel rejected because you did not love them enough to stay for them.
It is one of the more unique aspects of suicide vs natural deaths. My final message came in text form. He obviously did not want to wake me up at 3 am in the morning. Probably wise as I would have intervened. Not because I am an evil asshole pro-lifer that does not respect his desire to die. But because I loved him and had optimism and hope that people can be helped out of their darkest depths. That may be their view was distorted, or there were avenues yet to be tried. Or that maybe there were ways to find a meaningful quality of life despite difficulties. That and on some level I am a selfish human being like everyone else on the planet, who does not want to lose whatever seems valuable or integral to your own being. I suspect that is more so with parents.
For me personally, there is part of a prior text that I do not understand and prior conversations which now take on new depths of meaning. All things I spent an inordinate amount of time ruminating on as I grieved. I have no capacity to ask him the relevance as he is gone. So I am just left with the nag of what does that mean? I think every sentence increases that risk and text also does not convey tone very well.
The mobile phone in question also became an issue because I kept it and still have it in a drawer to this day. Takes me right back to the text I did not wake up for and the sentence I can never resolve.
But the sharp initial pain of loss does dull and the world keeps turning but memory and questions don't go anywhere. Now years on I understand why and his reasoning was sound but still sad, but regardless that changes nothing as he is still gone and it hard not to miss what you loved.
I have no idea if leaving an explanatory note is better or worse. It is a tough one. It will be unique to the individual what benefit or or harm it may do.