One easy way to achieve fullscreen images (using the featured image) on secondary pages (pages that are not the homepage) is via a Page Builder. You would go into the properties of the row and make it full-height. Should work with any theme, but you may want to consider a Page Builder Theme too (like Astra or Beaver Theme).

Here are 3 Page Builders I suggest, two of which have FREE versions:

Elementor: https://elementor.com/

Beaver Builder: https://www.wpbeaverbuilder.com/

Architect: https://thrivethemes.com/architect/

Since you are not a PHP Developer, you will find these Page Builders will make your life easier. I suggest backing up your website before testing anything out.

Hope this helps.

P.S. Nice Fall leaves on the homepage!

Regards,
JD

I will check those builders. I tried one, year ago, different one, but it was not usable.

I am lucky my GF is much better photographer than me, so I have tons of nice pictures to use…:D

Peter