I don't really know if this has been already discussed here, so I'm sorry if its already known, but I couldn't find it anywhere so I'm just going to publish it anyways, I have found another way to change the shiny odds of the game to any % you want (more or less), and this time it will be x/256 instead of a maximum of 1/256. (Note: The shinies will look shiny in your game, but if you try to transfer them, they won't be shiny.)
(As I said it's not mine I found it on whackahack made by an user called Mikelan98, in a post called "Cambiar la probabilidad de los shinies desde el 0% hasta el 100%")
In this case you will need to search in the arm9 for:
09 4B 0A 04 19 40 03 40 00 04 1B 0C 00 0C 09 0C
58 40 12 0C 48 40 50 40 08 28 01 D2 01 20 00 E0
00 20 00 06 00 0E 70 47 00 00 FF FF
and replace it with:
(change XX with the hexadecimal value you want in x/256) (50% for example would be 80).
0A 04 09 0C 03 0C 00 04 00 0C 09 0C 58 40 12 0C
48 40 50 40 00 0A C0 46 XX 28 01 D2 01 20 00 E0
00 20 00 06 00 0E 70 47 00 00 FF FF
And that's basically it, again I'm sorry if this post has already been made, I just couldn't find it, and I found it in a spanish post, so this could help others that don't speak spanish or that haven't found it yet I guess.
(Btw I tried importing the arm9 but it didn't work, I ended up using the Compression option, and that worked.)