>Any script or service you used can only access the most recent 1000 comments in your history.
This isn’t quite 100% true. Shreddit supports getting its comment list from a GDPR request, and if you do that, then it CAN access the older comments and delete them, it just doesn’t have any other way than that data request file to know about their existence.
You need to specifically use a GDPR export with shreddit though, it’s not magic, and if you don’t have a GDPR export, it can’t get around the 1000 post limit on the profile.