There are utilities (eg. andrewbanchich/shreddit) and even third-party websites that allow you to delete all your posts and comments (it's of course way too impractical to do so via the reddit website/app).

IIUC it is not possible to delete all one's posts/comments via the API alone (you can only delete the last thousand or so - IDK if repeating the process helps), so with most (all?) of these utilities one can provide a GDPR export of all comments/posts and they will then be able to delete everything.

You can request a GDPR export at www.reddit.com/settings/data-request (IDK if this works regardless of where you live), but it will take several days/weeks (I requested mine last week and I'm still waiting).