In my case, I could go to the ICO and register a complaint with them if you live in Europe then you can try this page: https://edps.europa.eu/data-protection/our-role-supervisor/complaints_en

Outside of Europe, the UK and California you probably won't have much luck to be honest, as they do not legally have to comply with you if you are not a citizen of any of those places.

As for if Reddit tells the EU to go away, that is suicide, whilst 49% of users are in the USA the rest of the 51% are everywhere else, that would be a very bad look for investors if the website was blocked in Europe and it would look especially bad for the company to be fined for gdpr non-compliance. So they would be in worse not to comply... Not to mention the loss in ad revenue. Nobody wants to serve ads when only half the site is going to see them. USA won't be much affected but anyone in the EU will.

They could also increase the fine, and if they still refuse to pay, sue them and seize assets. Either way, a massive headache