I tend to experience the following, could be coincidental but here it goes:
When you get your combo on your Pokémon, leave the area completely and go to a different place where it spawns. Shiny will pop up almost instantly with lure (and shiny Charm)
The shiny could be any Pokémon of course. But I have spent HOURS in one spot, chaining, chaining, chaining, and unless I am entering a NEW area - then a shiny comes up almost instantly. Sometimes it’s not the one I’m chaining.
After getting a combo and breaking it on a new chain, I would find shiny Pokémon around catch 20. So it varies. But try the method above?