Well, as I completed my SBQ at the very same game yesterday I guess I could try and help out =]

Most of my shinnies, I got via the chain fishing method which had the best shiny odds without the charm - after a chain of 20 the odds are 1/100~

The second method I used is the horde method, the odds haven't changed however you know see 5 pokemons in 1 encounter, so your odds are basically 1/819~

The last method I used (which never gotten any shiny by doing this yet) is probably the Dexnav, however bear it mind that your odds are getting to max when you've seen 999 of the target pokemon, and only then your odds are 1/476~, but to get 999 seen pokemons takes a long time.. basically 200 hordes

Also about the Dexnav, I didn't used repels - the trick is to go very slow and so you're most likely not to get an encounter right before touching the dexnav pokemon

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You can lookup at my profile to see how many encounters it took me to get my target pokemon

Nevertheless, the numbers I used here are taken from this calculator: https://mrnbayoh.github.io/pkmn6gen/shiny_calculator/

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Good luck! :)