Did you take permission from clients to send messages to them? You will run into legal trouble if you don't, and the service provider will send the authorities all your data.
I found some SMS Gateway BTC using Google.
https://www.smsgateway.to/en/btc https://www.ediarosms.com/i am not using it
Have you tried smapva?
If not yet check them first you can rent a number that could send and receive SMS and they also accept Bitcoin as payment.
smspva service is limited service you can use it for personal activity, there are no way to send SMS to thousands of customers.