Update:
- 2-2-2. ▷ Toggle subtasks:
- Increased stability in List view
- Dropped support of Board view
- No change in Timeline view
Get the latest version from Asana Bookmarklets 
Thank you @Parker_Aldric_Mar for the contribution!
I confirm the following code works perfectly 
I changed the curly quotes to straight ones.
javascript: (async function() {
let button;
while ((button = document.querySelector('.SubtaskGrid-loadMore'))) {
button.click();
await new Promise(resolve => setTimeout(resolve, 500));
}
})();
However, I believe no action is easier than one click, so I’d still recommend the Asana Load More extension (installation steps) rather than adding your code to my bookmarklets list.