Custom Theme from HTML Site – Correct Steps?

I am intending to convert a HTML site to a Custom WordPress theme, roughly following Option 1 described here: https://youtu.be/CZRgvAtwpxw?t=125

My current HTML site has multiple similar pages, but about 4 types of pages, each with slightly different content.

Is this a logical method to build the Custom Theme?:

Before doing anything with WordPress create one large Template HTML (.php) page with all possible sections.

Create my new theme with only that template.php file (and associated) .css file.

Then from within WordPress, duplicate that page for each new page and delete the unnecessary sections from each page?

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Is it straightforward to then edit my new pages with the Classic or Gutenberg editors and to use various plugins to add features – eg a call button or forms?

What do I need to do to make the same or similar template.php available for posts?

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Feel free to correct any assumptions I have made or to recommend a better tutorial.