I am hypothetical from curiosity. Delete my post if I am going too serious.

I figured odorless lethal gasses would be effective too if they get pumped into their car or house in huge amounts
Like pumping lots of carbon monoxide into a car

If they crash really hard it would be ruled an accident almost certainly, but if they die from poisoning itself then it will be found out that this carbon monoxide could not have been created by the engine... so a homicide investigation. I would fear touching any surface with my bare hands. They always look for fingerprints and they would check the entire car and all doors for fingerprints.

I think one of more effective and more undetectable ways would be an explosive homemade drone. Regular router frequencies to avoid suspicion like 2.4G. But it would open an obvious homicide case. Nobody uses this method. Explosives that get triggered by a force like crashing are easy to make but they are very unstable and would probably kill you before the target. I would not like to handle them personally if I had a choice. It would be better to make stable explosives and only attach a force detector creating a spak like revolver caps.

You can also make basic homemade triggers with matchbox, matches and a rubber band

It is made by cutting out a side of a matchbox which you rub your match againist for it to ignite

First you take that paper that you cut out and put a match in the middle of that paper paralel to the longest axis of that paper
Then you cover the match head with that paper folded in half tightly
Then you apply your rubber band around it as hard as possible, so that it creates a lot of force
Done

If you pull the match out rapidly it ignites as if you pulled out that grenade ring
You can do the reverse to the inside so that it would ignite like a landmine but it is much less reliable

I tried visualizing it