One enemy of the home cheesemaker is consistency. The big boys have it easier in this regard because there are dedicated quantities, large scales, and bigger equipment to handle the processes.
For us, as much as 1/16 more or less rennet or 5 degree in temp can produce a whole other cheese.
So for frozen buttermilk, it's more challenging to produce the same results. For one, initial buttermilk strength may differ between batches. For another, the time the cubes spend in the freezer may decrease viability.
In the end, these two things can be mitigated by measuring pH and adjusting your process accordingly. But it's still easier to just use the DVI cultures. If you buy the bulk pack and do smaller batches, I would measure the bulk pack out and separate into equal parts so that you can have 1/2 DU or less in a single pack.
For the cubes, use more like 3-4 per gallon.