Have you seen webpack-merge? I designed it to make it easier to configuration. The basic merge is enough in practice and it allows you to extract commonalities and even compose if you want.
I agree a single file is a great starting point. I prefer to maintain my own "blocks" for the reasons outlined above. In part it's about owning your configuration as you can't wait for third party fixes. That said, if you own your configuration, it can be sensible to package it up so that you can share it across projects (middle ground).
I wrote a little free book that digs deeper into the way I think about webpack. I hope you find something useful there.
+1 for the webpack-merge, I've been using it ever since I found it. Much more clearer than isProduction inline hacks.