I suggest keeping the 6-digit codes relatively broad, first of all to avoid FoRs becoming extremely niche and secondly to avoid overlapping too much with other (non-engineering) fields of research. I.e. inclusive design and human factors are very established areas of research and expertise within HCI and product design. Researchers working within these spaces will likely identify more closely with codes such 120304 Digital and Interaction Design or 120305 Industrial Design.
Environmental engineering has four broad 6-digit codes. Taking inspiration from these, I propose:
Humanitarian