Hi gunnar; you can definitely reset your Spectre secret at any time, but you're right that doing so will change all of the passwords on your sites after you log in.
Imagine someone finding your phone and trying to sneak into your Spectre app, they could reset your secret but they will never find your original site passwords this way. That's because your secret is your proof of identity and without it there is no way to get the passwords that belong to you.
As such, resetting your secret will require you to reset each of your sites' passwords back to their new passwords.
That said, if you reset your secret and later remember or find your old secret, you can reset your user back to the old secret and restore all of your sites back to their old passwords.
I always recommend using a secret that is a simple sentence, perhaps it's silly or funny, and it will stick in your head much better that way while still being very long character-wise, so high in entropy. It may also help to not turn on Face ID for a few days or weeks so that you're forced to type in the secret manually and get it into your fingers.
All of that out of the way, are the issues you're seeing with Face ID temporary (eg. Face ID sometimes doesn't work, but other times it works again) or does it turn off permanently? Does it still work in other apps or to unlock your phone? Can you tell me more about the issues around Face ID you're experiencing if you feel there might be something wrong in the way Spectre handles it?