Neat idea - using the keychain for master key storage is a clean solution for the solo developer case.

If the team sharing and multi-environment side ever becomes a problem, we've been working on something similar at envmaster.dev that might be worth a look.

You'll probably like https://varlock.dev (free and open source) Im just about to roll out similar built in secure-enclave encryption with fingerprint unlocking. But integrated into a larger tool that does validation, type generation, secrets protection, and a bunch more cool stuff!