Hi there,
Since you've made progress with tonification, we should probably build on that rather than change gears. One thing you haven't addressed yet is Wind invasion (of the Tai Yang channels) causing intervertebral disc disease or stenosis, leading in turn to the hypermetria by causing an upper motor neuron lesion.
Du Huo Ji Sheng Tang leaps to mind, given the slow success of the Kidney tonic, but if you would like more Spleen support, then Yi Yi Ren Tang (NPHC) is perhaps a better choice. It addresses Spleen and Kidney deficiency, but Damp Heat as well, while expelling a Wind pathogen. In a large dog with very weak pulses, it is my first choice.
XCHT/BH&K would normally be indicated by a full pulse, and is better suited to CCL tears, hip dysplasia or sacroiliac dysfunction. It might not be a good fit here, given everything else you've noted.
Also ensure the dog gets a chiropractic evaluation. They really are mandatory in any UMN lesions, as they can take pressure off the cord immediately, producing prompt gains in mobility. Look for an improvement in 48 hours if it's going to help. If it does, let the owner know it should be a regular thing. Continued use of YYRT should slowly reduce the required frequency of treatments
Hope that helps!
Steve