I don't know any good ready-made applications for this. Our company ordered the creation of an application from these guys
https://www.infopulse.com/services/s...n-development/. They created a really good application for sending messages. The application works both on the computer and on the phone. The application can be configured to send messages for a certain time, you can create different groups of contacts. The message text can be changed in this application. It is very convenient.