In search of stable auth library for CI4

What do y'all think, are there any good alternatives to Myth Auth?

Tailored for CI4, no.

Or, is Myth Auth close to ready for CI4 prime-time?

As with all software, it will be done when it's done. ;-)
You can kickstart the development if you want by solving the current issues or just using it, and finding bugs.
https://github.com/lonnieezell/myth-auth/issues