You may not have to. There are numerous reports that Reddit is restoring bulk deleted comments...

You could consider making a data request first and after they respond, make a deletion request when you’d like your account removed.

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.

https://www.datarequests.org/sample-letters/