I think the only way to truly delete anything from reddit would be living in EU and enforcing a GDPR request, but even in that case, I believe it would be very difficult to check they actually comply.

Wouldn’t work. GDPR is not copyright. Deleting the username is enough, unless you have doxed yourself in some post.

Rather, it can be argued that GDPR requires restoring comments at least in some situations. Comments may be necessary context to understand replies or even other posts.