You could go for Paracetamol/Acetaminophen. The lethal dose is quite low and in theory low enough someone could poison someone else with it. And once symptoms set in people’s livers are often beyond rescue and they die a very gruesome death unless we find a transplant organ in time.(And even then survival is not guaranteed)

And it’s relatively save to use in writing as it is coated/mixed with enough bittering agent these days that it actually wouldn’t work that well to secretly posing someone.

If you need something with a shorter timeframe Methanol is an option.

And of course there’s always Dihydrogen monoxide-everyone who has even had one drop of it will die eventually but the time range depends on the dose. With very high doses people die in minutes,with medium doses (this is actually sometimes used by inmates to kill themselves) they die within a day, with lower dose after decades, but some die mere days after they ingested the mere last drop of it. Nasty stuff and very available.