Can you be a steward of your own codebase or only someone else's?

Yes, you can be an open-source software steward of your own codebase or someone else's.

Ownership or authorship of the code is not a determining factor for stewardship under the CRA. What matters is whether a legal person systematically provides support on a sustained basis for the development of specific free and open-source software intended for commercial activities, and ensures the viability of that software, as defined in Article 3(14).

For example, a company that develops open source software for integration into its own products, then publishes it separately without placing it on the market, can be the steward of that software. Similarly, a foundation can be a steward for projects it hosts and supports, regardless of whether foundation staff originally authored the code.

A legal person can even be a manufacturer for one version of software (such as a paid edition) while simultaneously being a steward for another version (such as a community edition) of the same project. See Can a manufacturer also be an open-source software steward ? for more details on this scenario.