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

Supabase

How can I set up an admin dashboard with supabase-js querying to bypass RLS stuff?

You'll need to use the key from project Settings -> API -> service_role secret

But never use it in browser, this key should only be used with a trusted server-side app.

Yes, but how can I fetch data with the library and bypass RLS without exposing that key?

You'd need a server-side component that prepares dashboard data.
I don't know what's your architecture, the options I see are:
1. allow admins to see all data they need for dashboards,
2. prepare dedicated "security definer" functions that expose dashboard data (and check that the current user is an admin)