yah, that's the kind of thing subgrid is meant to solve. in my head, the structure looks more like:
<body class="content">
<header class="popout"></header>
<main></main> <!-- implicitly in the content track -->
<footer class="full"></footer>
</body>
you shouldn't have to add an extra div inside <main> to be able to use the parent grid. so you subgrid on <main> and get the benefits of just adopting the parent grid for popping things out, rather than nesting with extra divs/classes.