Is using nested extends a wise idea or will it be a performance/maintenance nightmare?
I like to have the document skeleton with the head, scripts etc. in a _base.html template and if there's more than one basic page layout (think sidebars etc.) I additionally extend it with _layout-a.html, _layout-b.html, templates. Everything else I use includes for.

The templates are being transpiled down to PHP and cached, so there should not be any measurable performance influence really.