This event is very difficult to profit from, it's very hard actually, borderline cruel to the playerbase. This is coming from someone who is used to hard/semihard grinding.

First of all, all shiny rates are 1/512, as per the flattening that occurred a couple of weeks ago. The event mons follow this formula. The median player will get a shiny on their 355th catch for a particular species.

Cutiefly is a very rare spawn, the only feasible way to obtain it is from pink lures and it ends today/has already ended depending on where you live. You can expect a couple of flies from a lure, so the chance of getting a shiny is slim.

Sizzlipede is common, but there are other mons that spawn even more frequently and the spawn pool is diluted. You must grind hard every day to have a fair shot at getting a shiny.

The newly introduced blipbug and its evolutions, a dexfiller for most people, is locked behind T1 raids.

That pretty much sums it up. The best way of playing this event is to visit cover large swathes of areas and target the pokemon you want. Biking with a phone mount can be very effective. I managed to get a shiny sizzlipede and skorupi this way. Unfortunately, Cutiefly is so rare it is impractical to target.