I think shreddit.com is based on someone maintaining and fixing the old python open source shreddit script on github. Both that version and the new rust shreddit support using the GDPR archive files I believe for free, you just have to run it yourself.
The issue is that I may not get my archive file untl after July 1st, at which point I'm worried that these scripts will stop working due to the API changes. Even $15 on shreddt.com may no longer be an option.