Indeed, I also cannot come up with a better tagging than you've already proposed, and it seems there is no way to uniquely denote diffraction methods other than via the x-ray-based ones.
As I see it, there are two ways this can be sorted out:
Add a new specific tag
We can introduce a neutron-diffraction tag, or something more generic, like a neutronography or neutron-instrumentation tag.
In my personal Zotero library, I use separate tags for instrumental methods and the classes of underlying principles (e.g. effects of which particles or/and at what wavelengths).
Following this logic, I'd probably split x-ray-diffraction tag into diffraction-methods and x-rays tags.
This way one can cover x-ray/electron/neutron diffraction as well as photocrystallography by supplying a set of two tags, e.g.
The main advantage of this tag classification is modularity and flexibility; tags are less narrowed-down and can be re-used, e.g. microscopy + neutrons or scattering + neutrons.
The drawback is that there is a five tags limit per question, and one method already would occupy two slots out of five.