I usually don’t copy the function.php file, I just create a new function.php file for the child theme with whatever changes I want in there.
So if you need to make changes to say the header.php you can copy the header.php and make your changes in the child theme.
As the child theme files are pulled first.
I will use the page.php to create a new page template.
I don’t understand your being lectured ‘not’ to copy template files from parent to child. ? anyone??
If you want to modify a template, the only way is to copy from parent to child!
I’m not sure who would have lectured, it’s the only way to do it since templates are not usually full of hooks, etc to latch on to
Of course that rule doesn’t apply to functions.php, but we are all aware of that here!
I almost always have to copy header.php to my child to modify layout. When a new version comes out, I just run a diff to see what I need to update in my child version. Or copy the new header.php and redo my changes