Hi @enxaqueca
I don’t know regarding caching and optimization plugins, but what I do know is that the Async and Defer setting is broken after a WP update that changed some internals. (probably to protect some abuse of other WP developers messing with things around)
I’m sorry, but I can’t help you further. However I can as a developer and experienced WP user recommend two caching/optimization plugins.
The best free or low price caching/optimization plugin is WP Fastest Cache: https://www.ads-software.com/plugins/wp-fastest-cache/ and if you really have an important website I would really recommend WP Rocket.
WP Rocket are absolutely the best of the best if you’re serious about caching, optimization and the whole shebang.
And I think both plugins have an option to defer and/or async everything or specific scripts (not included in the free version of WP Fastest Cache, also in no free version of any caching plugin. I searched and tested a lot of caching plugins for work).
Kind regards,
Remzi