Hi Dev Dex,
Actually you can't replace any input field with custom editor in Portal even if multiple line text field.
Because when an entity form is rendered by System, every input fields will be binded with some special attributes, these attributes are used for validation and submission,
system won't recognize our own element. (You can open inspector to check these attributes).
So we should instead load rich text editor on a blank element(just add CDN libraries in Web template source code), then use javascript to populate default multi line text field with content in rich text editor,
when everything is tested well, hide the default input field with js or css.
E.g:
When I leave rich text editor, description field will be populated content in the editor.(For presentation, I don't hide the default field)

Web template code:
{% entityform name: 'My Portal Contact Form' %}
You can use it for your own entity form for test.
Another thing which we should pay attention to is that content in rich text editor is formatted as HTML code,
we need regex to match HTML tags and replace them with corrensponding symbols.
e.g:
Clofly
Mao
is actually
<p>Clofly</p><p>Mao</p>
In my demo, I replace <p> with ""(blank space), replace "</p>" with "\n".(new line)
You could add more regex to do converting work.
e.g:
1. Dynamics
2. 365
is actually
<ol>
<li>Dynamics </li>
<li>365</li>
</ol>
I'm not familiar with regex, but there are many existing samples, just add them to final line of my source code.
Regards,
Clofly