Personally, I think the solution you propose is too complicated for what you want to achieve. Here I will leave two simpler alternatives that even people without technical knowledge can use:
Option 1: Use plug-ins / extensions in the browser
In the different browsers you can find plugins and extensions to add functionality to the text editor of GMail mail. As in the question you talk about Chrome, I'll give some examples for that browser.
In the particular case of crossing out a sentence ( strikethrough ), you could use the extension Strikethrough , which does just that: add a button to cross out text in the editor.
But you can also find others like TeX for Gmail , < a href="https://chrome.google.com/webstore/detail/fix-compose-for-gmail%C2%AE-to/hpoidnelefpoofhdioielagfklahfcjg?hl=en"> Fix Compose for Gmail or Classic Gmail Compose , to mention a few in Chrome.
Option 2: Copy-paste from a text editor
Why complicate life with editing HTML or installing plugins / extensions? Here I leave a supersimple method that works even without having any knowledge of HTML and without knowing how the extensions work.
A method that even my grandmother would know how to use:
Write the email in your favorite text editor (Microsoft Word, StarOffice, WordPad, Google Docs ... it does not matter), even with options not available in GMail: strikethrough, superscripts, subscripts, horizontal lines, etc. .
Copy the text to the GMail email editor
Tachán! It's done
