Beware that when you actually do send the bulk SMS messages the service provider(s) may block the message if their systems perceive it a spam.

There may be some way to register in advance or forewarn them prior to actually sending the messages.

There may even be an existing infrastructure to submit bulk addresses with the messages, probably a service primarily intended for public services and emergencies.

You don't want a solution that only works when you test it at small scale but fails at full load, or only works once at full scale before it gets blocked.