What is the character limit on the HTML field for email notifications?
The character limit is 65,535. If the text length of the HTML code you've provided is lower than 65,535 characters, then check the code; it could be that the HTML string is parsed and converted into an HTML document, which in turn increases the size of the final HTML value to > 65,535 characters.
Hence, since the HTML character count should be less than 65,535 characters in the code that you've provided, you could remove a few lines, like the commented lines, from the code (around 6000–8000 characters) and pass it. It will pass through.
Reach out to firstname.lastname@example.org if the issue persists.