Hi! I'm also Spanish and own a Keychron K2 Pro. For me using the layout "English EE.UU. with AltGr" and just press Alt + a = á, Alt + n = ñ is working fine.

Anyway in case you want to use VIA (very helpful if you want more custom macros), Keychron has a really good tutorial. After you load the JSON file you should see something similar to the end of the first video in the tutorial page. In the upper left corner you should have four layers, 0/1 are for Mac mode and 2/3 are for Windows mode.

Now some explanation of how layers works. You have the "main" layer (0 in Mac, 2 in Windows, from now on, will only refer to the 2/3 layers as you are on Windows). From what I understood you don't want to change the main layer as is the usual config. In your case, you want to modify the secondary layer (3) so when you press MO(3) + = , ( MO(3) is to tell the keyboard to change to the layer 3 when pressing that key). For that you should be able to map the position of the 'a' key in the layer 3 to the letter 'á', and make MO(3) + a = 'á'