This web page allows you to create customised versions of the GDPR "Nightmare Letter", which compels the recipient to disclose all data they hold on you.
Note that you will still need to provide the relevant identifying information to the recipient of this letter, but this should get you most of the way to making your very own nightmare GDPR disclosure request.
Everything you enter on this page stays on your browser. You can see the source code on Glitch.
Made by @Minkovsky