All greetings!
Tell me, please, is there any way to completely disable the TinyMCE?

You could replace your Content field with a new textareafield in the model using getCMSFields().

The only problem would be that the text you enter would not have any formatting, not even line breaks.

Yes, replace the TinyMCE Editor with a TextArea, just as yurigoul said.
It will keep line-breaks, if you set the Content field to 'Text' instead of 'HTMLText'.

Here's how your Page class could look like:

class Page extends SiteTree
{
	public static $db = array(
		'Content' => 'Text'
	);
	
	public function getCMSFields(){
		$fields = parent::getCMSFields();
		$fields->removeFieldFromTab('Root.Content.Main', 'Content');
		$fields->addFieldToTab('Root.Content.Main', new TextareaField('Content', 'Content', 20));
		return $fields;
	}
}

You can even use BBCode syntax for formatting if you use the following statement in your template:

$Content.Parse(BBCodeParser)

Update You'll have to run /dev/build if you change the class as shown above

My guess is that it only shows the line breaks in the output on the site if you wrap it in <pre> tags, unless you use BBcode.

No, line-breaks will get converted to <br/> if your field type is 'Text'. No need for a wrapping <pre> tag or the BBCode parser.

Thank you for your help! Your advice is very useful.