Thanks for bringing this our attention. correct me if I am wrong, I believe this is the ACS documentation you were referring to?
you should be able to send messages with 260+ character count. Azure Communication Services supports sending and receiving of long messages over SMS. As the doc mentioned, "some wireless carriers or devices may act differently when receiving long messages. We recommend keeping SMS messages to a length of 320 characters and reducing the use of accents to ensure maximum delivery."
But we will check internally with the engineering team to confirm your scenario and will get back to you when we hear
back from them.