Identity provider recommendation for B2C marketplace SaaS

I'm seeking feedback from the community on a good cloud SSO platform for my startup. The Auth0 Startup program looks promising, and I've had good experience with them in the past, but I know they can get expensive at scale - especially if there are a lot of free users. Firebase is my go-to for mobile apps, but this will be web-only. I've also used on-prem Keycloak and while powerful, I'd rather outsource identity this time.

For reference, here is my anticipated use:

A web-based (no mobile) B2C marketplace SaaS platform deployed on Azure or AWS as a containerized docker app. On the business side, I'm expecting <1000 "companies" that may have multiple users under their domain that are managed by a company administrator. On the "consumer" side, I'm expecting ~50k individual accounts. It must support OICD and authenticate with the major identity providers (namely Google and Microsoft). I'll also need to support authenticated API access using long-duration API Keys.