Hello,
We really don’t recommend using subthemes. It’s better to customize templates or create template files in the client’s theme. Making a child theme can also be a good solution.
If you want to add pages to menus, you can edit the menu and add any link you like in the admin dashboard.
Front-page images can be replaced by images of your choice. However, this can only be done on the administration side. To do this, you need to modify the front-page image, which will then appear in the front-end.