Do you have an example of these "furniture panels" that are higher quality and require less effort than a mod to a fully-realized piece of furniture with easy instructions?

I left off "as cheap" because you didn't say it, but it's a huge part of the puzzle that you deliberately ignored, too. But I'm really interested in seeing these "furniture panels" that you're recommending.

In the UK at least, furniture panels from B&Q, Wickes, Selco, etc, are very much inferior to the stuff some IKEA (e.g. METOD and PAX) furniture is built from. That was why I used PAX carcases to create built-in wardrobes. Plain furniture panels would have been lower quality.