Any tips for what an EU citizen can do against this, beyond upvoting this story on HN?
I think it's too late. https://european-pirateparty.eu/parliament-approves-chatcont... https://www.patrick-breyer.de/en/posts/chat-control/
You can read the derogation here: https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX%3A... . There is a somewhat helpful first page that provides context.
Personally I think it is quite a reasonable derogation; the pirate party clearly disagrees.