I don’t think just having a picture of it will do it justice (I built a custom CMS, for example).
I don’t see an issue with displaying images of the work you’ve done. Take a look at WordPress, Statamic or Kirby. There are countless CMS sites that showcase features in screenshots. Perhaps instead of just linking to the site in question (if that is in fact what you’re doing), why not create a presentation out of it? You can create something similar to a blog post (what you see on the Statamic link above) where you mostly focus on showcasing the qualities.
Well just for the website I would be buying a theme and then over customizing it in WordPress so it would become my own unique custom theme.
I never understood this point of view. Unless you’re completely redesigning the theme, I wouldn’t consider that “unique”. That’s another topic, however. I do agree that technology in five years will be drastically different. I wouldn’t worry about the choice of CMS or any of those specifics right now.
I would start thinking and continue to think about making your services incredibly easy for clients. Set yourself apart from the others in a way that is outside of current practices. Be innovative. Unique.
@losrack What you could do is make sure your child theme css is below the theme css in your head section.
<!-- This is your MAIN theme CSS file -->
<link rel="stylesheet" href="http://yoursite.com/path/to/main.css">
<!-- This is your CHILD theme CSS file -->
<link rel="stylesheet" href="http://yoursite.com/path/to/child.css">