[burnt-resistor]

Use a template-based configuration management system to pull in secrets from a secrets management system (passwords/credentials/PKI management). It's an antipattern to add even more complexity by bolting on configuration or secrets management as part of nix, systemd, or anything else that's OS- or distro-specific. Package managers, and OSes need to offer sane defaults but then have sensible boundaries and allow configuration responsibility to be delegated to users and/or configuration management.

[eviks]

How would that help if the result of the template is the same single config file that doesn't separate concerns?

[burnt-resistor]

Facts sources, logic, and secrets are typically best using logic-less templates. Good configuration management systems are sufficiently understandable, flexible, and maintainable.

If a file is static, then most configuration management systems allow dropping in such too without templates.

What specific question or concern do you have?