It can be done with shreddit: https://github.com/andrewbanchich/shreddit
It has a built-in option to point it to your extracted GDPR data, and will edit and/or delete all the comments that are listed in there.
Just configure it according to the instructions, and then let it run unattended for a while. It took about 15 hours to delete the 13000 comments on my 12 year old account.