This simple script will turn all Greek non-GSM characters in GSM, allowing more characters per SMS segment. This will be the case for capital letters only
git clone https://github.com/evanTheTerribleWarrior/sms-greek-character-conversion.git
python3 sms-greek.py
You can use this link to check that your message is GSM: https://twiliodeved.github.io/message-segment-calculator/