Is it possible to combine several template files togather and make single integrated template instead?
In drupal 8 there's this rule called theme inheritance and backward compatibility, what you have to do to accomplish what you want is to make sure that your subtheme does not inherit any core theme's HOOKS

You might want to play around in your subtheme and set your base theme to null
I haven't checked it myself but it might work.

And then of course you have to define new preprocess templating rules in your themename.theme file using theme_suggestions_page_alter function, you can read more here

Then based on what you defined in your .theme file you may proceed with creating a unified template file

Also you might want to read this suggestion that somewhat explains a similar use case