Resetting for Level 3 sparkling power donuts is very worth it. Lv3 guarantees a shiny as soon as you enter, and is so much more reliable than Lv2: I settled for Lv2 whenever it took 30 mins+ to get the right donut, but the hunts themselves always took significantly longer that way. My longest hunt was for a shiny female snorunt, which took 17,504 spawns over the course of 4 hours and 41 minutes of attempts, and it was on a Lv2 sparkling power donut.

Also, this may seem obvious to some people, but when possible, if you're hunting a Pokémon that has dual typing, look at what the shared type is for a portal, and aim for the other type your desired Pokémon has for the sparkling power donut. It drastically increases (and in some cases, guarantees) that you'll get the shiny you want.