You just need a higher Sweet Flavor Score. Flavor Powers are largely random, as the same recipe can produce differing results, though it can be observed that there are probably thresholds that must be met before certain Flavor Powers can be rolled. There is currently no concrete (i.e., datamined) information on the precise rates or requirements for how they are selected, though from player experimentation, Sparkling Power appears to require some higher quality donuts in order to appear compared to some other Flavor Powers.
Given that there are some circulating anecdotal claims that 420 is the minimum Sweet score needed for a chance to get Lv. 3 Sparkling Power, it seems at least inefficient to use your Sweet berries to make donuts below this score when you could save them up to create more reliable donuts. I wondered if it's possible to at least get Lv. 1 Sparkling Power, so since I had a bunch of extra berries on hand, I made 20 donuts each containing 4 Haban Berries (340 Sweet) as a quick sample for you and none of them had Sparkling Power. On the other hand, although it wasn't guaranteed, I had no problem getting Sparkling Power of all levels when using 5 Haban Berries (425 Sweet). This isn't much data but it implies that it's either impossible or at least prohibitively rare to get any level of Sparkling Power under 420.
In any case, you should hold off on hunting for shinies until you have access to recipes with 8 berries, since nothing you can create before then will be efficient and you'll mostly just be wasting your time and your good berries. Plus, you'll unlock more possible areas as you keep progressing through the story, which opens up more potentially desirable shiny hunting locations.
I also recommend this video for a good spoiler-free analysis with a larger sample size, which concluded that the single most efficient strategy is to focus on donuts with 8 Tanga Berries for the highest possible Sweet score of 760, which outpaces the strategy of creating a larger quantity of donuts in the 420 range.
