First of all, sorry you find yourself at that point.
I've also dealt a lot with the guilt and uncertainty of how my death would impact my loved ones, and how to minimize that pain. There are no meaningful solutions. The pain will be there and it will be inmense if you end up doing that, you probably know already.
If anything, closure does help just a bit, or rather the lack of it hurts even more cos it adds another layer of incomprehension and what ifs and whys and all that to the grief. So a letter I think would be best. Telling them you love them, being honest with the fact you are dealing badly with how this would affect them, that it was in no way their fault, and that you found peace... The comment above has good questions to answer too. It's all sad stuff but better be upfront about it. The only pain you could spare them is that of the whys, the rest of the grief would still be there.
However if you are still considering backing out, maybe a month is a bit short for figuring that out and specially if you still have doubts. Don't know your personal situation, but please try to think it trough and the reasons for which you might want to back out. Take it easy these days regardless. hugs <3