Woah, the new docs include this very thing
$activities = ActivityFeed::query()
->with(['parentable' => function (MorphTo $morphTo) {
$morphTo->morphWith([
Event::class => ['calendar'],
Photo::class => ['tags'],
Post::class => ['author'],
]);
}])->get();
https://laravel.com/docs/9.x/eloquent-relationships#nested-eager-loading-morphto-relationships