We integrated Auth0 into our product back when they were a cool startup. Since Okta acquired them they changed their pricing aggressively, which makes it very costly, especially for B2C that have many MAU. Furthermore, they removed MFA from their "cheapest" tier, which I find very unconsumery.
I know that there are alternatives, but our code is deeply integrated with Auth0 and shifting away will take plenty of time and be very costly as well.
Would be interesting to hear about others' experience with Auth0 and their pricing shift. How it affected your startup? did you decide to stay with them? or move to another solution (and which)?
Anecdotally, we've seen a number of folks choosing to migrate off of Auth0. Sometimes in a hurry, to avoid contracts renewing. The length and effort of the migration really depends on the particular applications, how they are using Auth0, and what features they need. Auth0, to their credit, still lets you export password hashes (unlike Cognito) so you can do a pretty transparent migration.
On the flip side, we've lost deals to Auth0 for valid reasons. So Auth0 can still be competitive. Have you talked to your Auth0 sales rep and indicated you are unhappy? They might be able to do something for you. (I understand that migrating just to end up in the same place, functionality wise, is frustrating.)
You mention being deeply integrated with Auth0. I'd love to learn more. It might take less time than you think, depending on the feature set of the alternative and how you integrated. The Authorization Code grant is pretty similar across providers, for example. And we have the concept of Lambdas, which is similar to Auth0 Actions.
Here's a migration guide I wrote: https://fusionauth.io/docs/v1/tech/migration-guide/auth0
It focuses on moving to FusionAuth, but the first half might be useful to you, as it is all about planning and getting data (esp password hashes) out of Auth0.
Finally, if you want to run your own auth server, FusionAuth has a community edition that you can download and run for free. https://fusionauth.io/download has more details, and I'd be happy to chat with anyone (my contact info is in my bio).