> additionally you are spinning up a full PHP interpreter when serving static assets.

I think that if you do this you are one RewriteRule away from doing it right.

> In my experience[1], putting Nginx in front of Apache to serve static assets and leaving Apache as a sort of application server

I also think that using nginx only for static content is wasting perfectly good resources for nothing. Apache can serve static assets just as good as nginx does. No need for an other layer in your stack.