I found loadScript (https://github.com/discourse/discourse/blob/main/app/assets/javascripts/discourse/app/lib/load-script.js#L50), but the <script> tag it adds doesn’t have the async attribute, and there is no way to add it. That’s kinda bad.
Edit: nvm, apparently scripts added this way are implicitly async… Scripts: async, defer.
TIL.
Eh, too tired rn, will get back to this sometime later.