https://www.reddit.com/settings/data-request
Having worked at a company that had a massive influx of GDPR requests we weren’t prepared for, this one could actually cause them some trouble if Reddit don’t have that process properly automated.
You could consider making a data request first and after they respond, make a deletion request when you’d like your account removed. This will use even more resources.
If they do not respond to either the data request or deletion request (or do not fulfil these requests fully), you can make a complaint with your local data protection office or the one Reddit is based in (maybe Ireland?). Make sure you invoke GDPR using the correct language for your request.
Here’s a template letter of how to do so under GDPR. You must request your data or the deletion of you data using the correct legal framework (quoting the correct legislation) and these templates make this easy. Plus they cover more types of data than just your posts and comments.