I don’t think separating read and write functions using different authentication methods would work — it makes sense from a security perspective, but I think the usability would suffer greatly.
Once you’ve passed the barrier of “unlocking” your own phone, you’ve intuitively stepped into the equivalent of your own personal space. Obviously that isn’t technically true, but for most practical purposes — and our own intuitions — that’s how it is for most people.
I’m not sure what the answer is, other than just sticking with passcodes. But presumably there’s always going to be a struggle between convenience and security, and bridging that divide is always going to be a major challenge.