Modifying to Portfolio Site

Hello Pros,

I would like to modify the SimpleGrid Wp theme

SimpleGrid

to act as a portfolio theme with subcategories.

The home screen at this moment contains posts. Those posts would be my categories.

Once you click on one of them, another page appears but not with a single post, but with a homepage lookalike page with the subcategory posts.

And when somebody clicks on a subpage category, then a normal blog page appears.

In other word:

I would like to have a subpage with the same pagination like the homepage, with my selected posts.

Example:

The first post is: Cars
When I click on it, a home screen lookalike page appears with:
blue cars, red cars, green cars (Like the home screen)
When I click on the Blue cars, the normal blog page appears.

Otherwords:

Clicking on an item (post) on the main page will forward you to a ‘sub homepage’ with the same item structure like the main page, then picking from an item will forward you to a post page.

I would like to aviod the Simplegrid theme + a portfolio plugin.

Can someone help me to do that?