Every customer you have done a job for is in the system already: name, number, what you did and when. That is the list. Nothing to import, nothing to build.
The reminders are written for the work you actually do: a service due, a season starting, a job that usually leads to another one. They go out as a short text in your voice, and anyone who replies lands in your inbox as a normal conversation.
When the diary looks thin, there is also a plain lever to pull: one message to everyone, or to everyone in an area, in a couple of taps. Not a campaign builder. A text.
