It won't be deemed a natural death unless you're expected to die in your sleep.
If you leave a note, it may help avoid an autopsy, but it'll be ruled as a suicide. Otherwise in almost all countries they'd investigate. An overdose on N for sure would be ruled as intentional.
I understand and empathise with the desire to make it look accidental and to avoid problems. There could be many reasons for it. In the PPeH, detectability on death is discussed for most methods - but unfortunately any blatant unexplained circumstances make an autopsy probable, which makes nearly all of them detectable.