Does that mean that I have to now user the API (Auth0 Management API v2) directly, even though I have created a const auth0Manage = new auth0.Management instance in auth0js?
Can I at least use auth0Manage.baseOptions.token or do I have to fetch a different one?
You are not going to be able to (or want to) give your client side app the ability to get all users from the management api. You will want to do this in your backend. Take a look at this FAQ for some more info about why: How do I use the Management API in my single page application?
You can also get all users without a number limit using a long running export job, which will return a file in JSON or CSV format: Auth0 Management API v2