Hey, we've had implemented Firebase auth in less than two days. Their doc is amazing and I don't understand why you think that it will take more effort than Auth0. Prices are really good (free, except if you use more than 10k/sms month).

If these three are your options, I would recommend going with Auth0. They have all functionality available as developer API (Okta e.g. not) so you can manage your instance with Infrastructure as code and can also easily add functionalities relatively easily with the API. They are also really powerful if we're talking about ABAC (Attribute based access control). You can also enrich your access token with custom claims from your MongoDB, that can be probably really useful, as you said that you're dealing with multi tenancy.

We're using Auth0 in combination with Fauna Fauna is a database, so it would challenge you're mongodb. But Faunadb is the first database that implemented a full end user ABAC system directly in the database. (And also a lot easier than the ABAC systems from Okta or Auth0). This helps us, to use Auth0 only as identity platform and doing all the authorization with enriched claims over Fauna. With that you can skip in a lot of the cases you're backend, and you can request directly from the frontend your database (Blazing fast). Also, you can replace in some years Auth0 a lot easier with some upcoming cheaper (Auth0 was bought by Okta for a hilarious price) and "easy to use" passwordless identity provider like Passage.id

I would recommend Auth0 only if you are willing to shell out money. You can keep up with their free version only for a very limited time and as per our experience as a growing startup where budget is an issue, their support was not very helpful as they first asked us to sign a commercial agreement even before helping us t o find out whether Auth0 fits our use case or not! But otherwise Auth0 is a great platform to speed up authentication. In our case we had to move to alternatives like Casbin for multi-tenant authorization!