I do not offer this service, but I did do a capstone on a similar use case.
I obtained my Pix4D certification for Pix4Dfields and they have several exercises that pertain to this.
The course taught how to use either a multispectral camera or RGB camera to create outputs.
Either could be used to help spot weeds since they will have a different spectral output as compared to the other vegeation.
In the one exercise an RGB camera was used with a TGI index to easily spot weeds in that particular use case, but results will vary depending on what you are looking for and where it is.
THe course then offered how to take the outputs to create a prescription map that can be uploaded to a terrestrial or UAV for the spot spraying.
If you are going to offer the spot spraying service, you will have to operate under Part 137 and this will entail another hurdle as well as meeting any and all state specific regulations and certifications for applying herbicides.