Injections give better control over the who and when.

What happens if just one jew slips it to the world, then you can easily find that coffee was poisonous.

Poison injections are the best delivery method.

Exactly, people underestimate how direct and efficient a poison injection is, especially that they book an appointment for their own slaughter. This means they can target specific categories by age, ethnicity etc... Also they could reduce or increase the lethal dose depending on your age.

Any other form is also not safe for them, some jews might take it by accident.

And it might be more than just a simple depopulation plan.