That’s a lot of pieces.
I’d probably do something like have the users in a group and plan to send to all users in the group. When a message got sent, you’d set a value in a user_custom_field and use some existing topic as the text and sender of the message. A job would run every so often and send however many messages you want. I’d put the group and custom field value in settings.
Then you wouldn’t need an interface or to create a new route at all; you could just use existing settings. And you could see what had happened using the data explorer, so you wouldn’t need to develop an interface for either.
But if you want to make some fields that people enter in the UX, you should check out the <Form> thing that I mentioned already.