Am I subject to the CRA if I maintain, but do not monetize, an open source project?

If you are the maintainer of an open source codebase, and you do not monetize it, then the CRA does not apply to you.

The CRA applies

only in relation to products […] supplied […] in the course of a commercial activity (Recital 15, emphasis added)

And it states that

the provision of […] free and open-source software that are not monetized by their manufacturers should not be considered to be a commercial activity (Recital 18, emphasis added)