Many cell providers have a mail to SMS gateway. Just sent email to the correct address and you’ll get SMS. As far as sending mail either with bash or with Python. That is quite possible and not hard.
Sorry I do not have the code at hand. Memory is Python standard library has SMTP capability and Bash you can just use the mail command. May have to configure mail on your Linux box too or just use a remote server.
Libreoffice has an HTML mode and also you can take any document and save as HTML.
Another way is use Markdown.
Another is to use a template editor like Bluefish. This is coding HTML but more easily.