Supabase is an open source Firebase alternative. Start your project with a Postgres database, Authentication, instant APIs, Edge Functions, Realtime subscriptions, and Storage.

Supabase

Is it possible to link additional (social) providers to an existing user/account? Can't seem to find anything in the docs about this.

Yes, if they share an email they are linked...
https://github.com/supabase/supabase/discussions/3098

I'm going to add a post from today... Not so sure on this with an email/password login in the mix...
https://github.com/supabase/supabase/discussions/5827

Thanks, will give it a look! 
I see that it’s common case to rely on the same email but that might not be the case nowadays. Especially like for myself, I sign up with Hide My Mail on all the places so it’s different everywhere.

Also because you noticed that on your issue, did you get Discord login to work on Localhost? Didn’t work when I tried but maybe I just have to give it a fresh look, was kinda in the rush 😅

I did not want to mess with any localhost issues when testing redirects so just install in on a "test server" on the net when doing that kind of stuff.