Yeah, I always hated that you couldn't set up email/password authentication. It's a stupid idea to use only third-party IDPs for authentication. People usually have multiple accounts with Google or use different emails for Twitter, LinkedIn, etc., which they want to keep from sharing with other apps. And I'm supposed to remember which account I used to log into a specific app?! No, thanks.
Once you start adding Google, you'll inevitably be forced to support apple sign in and all the others.
In my past job, we couldn't be bothered with all these big tech politics, so we actually disabled all the third party signups. We would require phone numbers because our value proposition was validating that users were real and easier to ban. Apple was not happy with this and their Apple sign ups allowed more anonymity, just a pain overall. We couldn't enable the others on Android either, because they'd be able to sign up with FB on a Samsung then can't do it on an iPhone, and that was just messy.