Don't worry about google pushing their authenticator app, other TOTP apps work fine.
I have not tried Authy on GrapheneOS, I know Aegis (available on fdroid, droidify etc) works fine and supports import/export of tokens. Looking at Authy's support pages I found
which should be able to guide you through the process of backing up and syncing Authy on Graphene so you won't be locked out of your accounts. Just a word of advice, double check that all your passwords are saved or backed up correctly and will be accessible after a phone reset.