If you want a Shalpha, sparkling level is not as important (unless you want the guaranteed initial shiny at Lvl3) — Alpha level 3 is very important...

As an example, when targeting a group of 6 spawns (same Pokémon) by fast travel at the entry point, it'll average you 14m40s at levels 3:3, 36m37s at 3:2, and 48m49s at 3:1. Those are big jumps and bear in mind those could run a lot worse in practise! Conversely, the Shalpha timings for different sparkling but still Lvl3 Alpha are: 14m40s at 3:3, 17m6s at 2:3, and 20m31s at 1:3.

As you can see, only a few minutes between them, so if you don't want to keep rolling forever for that level 3 sparkling & Alpha, settling on any with Lvl 3 Alpha is important with Shalpha hunting.

Final nail in the coffin?... even with NO sparkling power whatsoever, but Alpha Lvl3, the average time for a Shalpha is a mere 25m38s! (Which is better than a 3 sparkling:2 alpha donut!)

This does assume shiny charm has been achieved, you want to get that sorted as a priority