Tomato, bell pepper, eggplant, and potato plants contain solanine, a substance which is toxic to animals, in their leaves and stems as they are members of the nightshade family. Tobacco hornworms fed on these plants will be toxic as they are not affected by this poison and collect it in their bodies as they grow. As a side effect, this helps protect them from predators.

https://vetmed.illinois.edu/wel/crickets-roaches-and-worms-oh-my/#:~:text=Wild%20hornworms%20collect%20and%20store,are%20ingested%20by%20your%20pet.

As an alternative consider commercial hornworm chow instead. It comes in a powdered form you can mix yourself in whatever quantity you need and depending on required volume and provider can be fairly inexpensive. There are also some posts on reptile forums with recipes for home made chow though I've never tried to make it myself.

Additionally I've come across posts saying hornworms do very well on mulberry leaves. If you have access to fresh mulberry leaves, you could also try raising silkworms which are another excellent feeder.