Transformations having svm impact in battle right now is mostly just a stat injection. It's not easy in how to fix them to make them more impactful and not just "I've got moor stats now".
I have a suggestion that is not wholly hammered out, but perhaps it can be expanded on. Simply put, transformation should put extra stress on your body. Fighting at length with it ties you out.
The longer any transformation is held past x posts increases the cost by .5% ki, ignoring ki mastery and ALL other forms of ki reduction. Thus continues until defeated or de-transforming. If transformation is turned off, x amount of posts later it resets (maybe like 4-5).
In addition, we could add an initial transformation cost. We can come up with that later or it could be ignored.
In this way, you have to think about how you use your transformation, and how long you hold it. It could evan be lower strains for lesser transformations and more drain for higher ones. Just a thought.
So with this system, regulated training becomes even more important if you plan to battle a lot. This means sp becomes a commodity as well. All transformations become useable to help with your ki costs.