Buddhism

is not monism.

Not one! Not two!

To extrapolate on that: Not not one and not not two, as well.

If you read the Heart Sutra, you can get the gist of what appears to be a paradoxical truth in the sense that intellectualizing the investigation contrasts the direct experience implied, which again, due to the nature of language and thought, is a duality-based stance, i.e., intellect and direct.

The same can be said for eternalism and nihilism. Philosophically, you can debate a middle way between them or that they are two ends of polar opposites or that neither is true.

There is no one Buddhism in the sense that there are many schools of thought and practice. However, if you look into the matter you can discover a basic set of core concepts and related methods that predominate.

Although it is usual for any ism to branch-off into various sects and schools, the Dharma is based on the idea of practicing skillful means, i.e., the capacity to adapt and relate the essential meaning in ways that are flexible and related to the listener/student rather than purely dogmatic. That is not always the case in some schools.