I just feel sadly rather unconvinced by Doctor and Yaz, which is a shame as I'd like it to be a thing that worked plot-wise. It did feel like it was thought of as an afterthought, not an arc that was in mind when Yaz was introduced (when she was very boring and flat for the first season but then got a bit more interesting) - you could see from early on, for example, the intensity of how the Doctor felt for Rose, but this seemed to have come out of nowhere, there's no sense of quite why the Doctor should fine Yaz so special compared to other companions, IYSWIM?