alexispurslane How stable and reliable is GraphineOS compared to Google's stock Android version?
It is both stable and reliable and is a fully production ready mobile OS, this is not a hobbyist project which many would use the misnomer 'custom ROM' for.
Does it support Google Fi and if so, how well?
Google Fi works well, it requires the app to be installed along with our sandboxed play services which you can read about here:
(This works with both eSIM and Physical SIM.)
Can it run 2FA apps like Duo Mobile and Google Authenticator?
Yes it can run 2FA apps like Duo Mobile and Google Authenticator, however you do not need to depend on such apps for TOTP as open source apps such as Aegis can likely be used to replace both.
Does Firefox (and by extension the Mozilla Lockwise password manager app) run well on it?
Firefox works however as far as Mozilla Lockwise I cannot comment to though as I have never used it.
Vanadium and it's in built password manager work just as well if you wish to transition away.
KeePassDX also works if you wish to control your encrypted database locally and be able to transfer it between devices, as well as using it's MagiKeyboard for autofill purposes.
Anything further please ask away I and many others in the community will do our upmost to assist.