Hi Llewanda
I like your general thinking on this case. In a dog this young, we have to assume the renal disease is inflammatory in nature. The increased body weight on a higher starch and fat diet suggests the dog is Damp, making San Ren Tang the best choice to start with. It might help with the crystalluria, too but I think just as important a goal is lowering creatinine a bit.
If you see some improvements, then the diet is an issue - not the protein content, of course, but the processed starch component. My own belief is that it has a pro-inflammatory effect on the kidneys, creating glomerular issues that show as azotemia; and tubular damage that creates the nidi around which calcium accretes. It is insulin that mediates this effect and by choosing a diet which results in lower insulin levels, you effectively turn off the inflammation, potentially causing both the azotemia and the crystalluria to vanish. I realize that may be a tough sell to the owner given the involvement of the other vet, but if you can achieve something with San Ren Tang, then you have support (in the form of a response to therapy) to make the change. If the diet change did not help as anticipated, you can always switch back.
To target crystalluria more, you could add in Dang Gui Shao Yao San. With its low content of Dang Gui, it is unlikely to have much of a renal congesting effect, even though we're suspecting a renal inflammatory pathology. Rehmannia Eight may be too much in the way of a renal blood flow enhancer at this moment, although a lack of any response to SRT +/- DGSYS would suggest it as the next thing to try..
It's no problem to use these herbs in tandem with the hypotensive drug. If anything they will make it work better, not worse. Hypotensive drugs are more important to eliminate, however, if you end up using Rehmannia Eight and a response is seen. In that case, the hypertension is systemic and adaptive, with the goal of enhancing renal blood flow, and you wouldn't want to get in the way of that.
Hopefully this all makes sense to you. Please let us know how the case goes!
Steve