In the function for shiny rerolls there should also be a reference to a chain bonus. Details here:Sword:
[500 Capture Count (on)]
04000000 01379660 52803E80
04000000 01379664 D65F03C0
[500 Capture Count (off)]
04000000 01379660 F81E0FF3
04000000 01379664 A9017BFD
Shield:
[500 Capture Count (on)]
04000000 01379690 52803E80
04000000 01379694 D65F03C0
[500 Capture Count (off)]
04000000 01379690 F81E0FF3
04000000 01379694 A9017BFD
Note: untested, this cheat patches the function to always return 500, should be temporary.
Pokémon Sword/Shield give you extra rerolls to try to generate a shiny based on how many of the Pokémon you've ever battled, and how long your current encounter chain is. Shiny charm still gives +2 if obtained.
— Kurt (@Kaphotics) November 14, 2019
We're not sure what increments your chain counter, but this is it! pic.twitter.com/k5rX7tTcUf
If we could get that to return 25 then in combo with the 500 Battle Count that would give the highest shiny odds.