A few features are missing compared to Firefox's native password manager. The most relevant ones for me are a) keyboard shortcuts, and b) credential fields that are more than email-password pairs (e.g. AWS login that requires an Account ID, IAM Username, and Password)

Proton is the service you need, from password manager to protonmail, and much more. This is the one, I will not go without it.

I used to use Bitwarden and the old Firefox version that would work on your phone too. I now trust Proton Pass for it. Integration of the alias emails is wonderful.