The API-based deletion tools usually have to be tuned to delete posts slowly enough to not trigger Reddit's abuse detection. Otherwise, they'll automatically undo bulk changes like that.
There's no way I can manually edit and delete all of my content with the snail's-pace reddit UI
This is, unfortunately, the only way to guarantee that your posts stay deleted. My account was 15 years old. I still log in every few weeks or so to go manually delete more comments. It'll be a while.