I like NodeBB's source, though it would be nice to have real documentation for it 😉 Maybe even something like consistent JSDoc comments?
Thank you for the kind words, by the way. I'll keep updating this post from time to time 😊
WordPress itself can handle lots of traffic if scaled correctly. However, if you're looking to do lots of custom features that don't have WordPress plugins for and require custom work, then you're probably better off with a custom product.