So when I deleted my account a few years back, i hade a Firefox script delete all my posts and comments.
Then i created a ticket for support asking to apply my right to be forgotten, according tomy GDPR rights and also a written confirmation that my content would not be recreated after deletion.
They will play hard but just say that you then will need their official information such as conpony name, adress, phone number, email, reg number and preferred communication method for your local data authority to contact them.
Just send a GDPR deletion request.
You can find ready made texts online. Just plug in your email/username and send it. Make sure to request a confirmation of the deletion.
I remember it using the API, which is supposedly disabled now. There are warnings on early Lemmy posts to use the tool before Reddit rolls out the changes.