I know how to make this work well, I've too much experience with Node ESM.

These are the issues I can help work through:

Do you have any downstream projects that import wazum in its current state? If so, I'd like to take a look at those too so I know how to make the changes. The reason I ask is because in modern times, there isn't really a need to support old formats like UMD, etc. We can start fresh. Modern packages only need to publish ESM, and new projects should adapt. (IMO). But if you require support for old environments, it will be more maintenance burden (and issues).

This will be a bit of a change, will change the build and the output files completely, so that they are as idiomatic as possible (as close to regular vanilla ES as possible).