Separate head from header, so header would be just the top nav.
Move header into header subfolder in template-parts.
Move footer into footer subfolder in template-parts.
Understand the support for WordPress menus. Try not to hard code it this time.
Understand wp_header and wp_footer.
Archive & content
Create a template file just to handle the heading (page-header); make all templates use this.
Create a feeds template to handle the while have posts logic; this is duplicated in archive, home, page and single. Personally I think what we have in vincentzh-theme is pretty good level of abstraction.
Create a content template to handle the shared parts of page and single.
Consider renaming single to post, since it only handles posts not other single types (pages, attachements).
Now that the majority of CSS is migrated over, it's time to consider the next steps.
What is my MVP? The goal is to create an opinionated but generic enough template system for all my future theming needs. If that's the case, the following items must be completed:
Lay out a plan of the potential type of themes I'm going to need, define the scope of this project (how opinionated / generic) would you like it to be?
Test a child theme in action.
Do we need to change any get_template_part formats?
Now that I've replaced most of the CSS and a large portion of the templates with my own from vincentzh-theme, I lost a lot of the flexibilities of the original Twentynineteen, for example, the support of thumbnails in posts. I suggest to go through the supported features of Twentynineteen and make a wish list of features I've deleted but would like to support (perhaps in a more personal / elegant way) in future.
Now that I've replaced most of the CSS and a large portion of the templates with my own from vincentzh-theme, I lost a lot of the flexibilities of the original Twentynineteen, for example, the support of thumbnails in posts. I suggest to go through the supported features of Twentynineteen and make a wish list of features I've deleted but would like to support (perhaps in a more personal / elegant way) in future.