go-json | Fast JSON encoder/decoder compatible with encoding/json for Go | JSON Processing library

Where I get confused is that in order to load my schemas before I've published them (which I want to do while running tests locally and during CI/CD), I need to use relative local paths

Your initial assumption is false. URIs used in the $id keyword can be arbitrary identifiers -- they do not need to be resolvable via the network or disk at the stated location. In fact, it is an error for a JSON Schema implementation to assume to find schema documents at the stated location: they MUST support being able to load documents locally and associate them with the stated identifier:

The "$id" keyword identifies a schema resource with its canonical URI.

Note that this URI is an identifier and not necessarily a network locator. In the case of a network-addressable URL, a schema need not be downloadable from its canonical URI.

source

A schema need not be downloadable from the address if it is a network-addressable URL, and implementations SHOULD NOT assume they should perform a network operation when they encounter a network-addressable URI.

source

Therefore, you can give your schema document any identifier you like, such as the URI you anticipate using when you eventually publish your schema for public consumption, and perform local testing using that identifier.

Any implementation that does not support doing this is in violation of the specification, and this should be reported to its maintainers as a bug.