How to send lots of SMS for less than Twilio?

Not spam, all double opt-in. Even with 10DLC it's too much $, even on cheap providers like SignalWire. Email to text is unreliable and can't do above certain volumes (unless there's some way to get whitelisted I don't know?)

How can I do this for cheaper?

Edit: at request of a commenter here are some numbers.

Pricing varies by carrier but with my carrier mix it is between $0.0065 - $0.007 for 1 message on average. I need to send ~250k each month for now and also need lots of room to send more. These prices are SignalWire which is the cheapest I found so far. Bandwidth looks like the same. I think maybe Teli and Telnyx are a small amount cheaper, anybody here has used them and can say they are good?