Changing the keyboard layout to input a character not in my layout

[carancholito]

I use two keyboard layouts, Spanish (ISO) and Polish. The character \ is not available in the Spanish layout, but it is in the Polish layout (in the "backslash" position). So drawing inspiration from #697 (comment) I made a complex modification that changes the keyboard layout, presses "backslash", then changes the keyboard layout again. This places a \ when doing right_option + grave_accent_and_tilde (the key to the left of 1), as I want.

{
"description": "Backslash",
"manipulators": [
{
"from": {
"key_code": "grave_accent_and_tilde",
"modifiers": {
"mandatory": [
"right_option"
]
}
},
"to": [
{
"key_code": "spacebar",
"modifiers": [
"left_control"
]
},
{
"key_code": "backslash"
},
{
"key_code": "spacebar",
"modifiers": [
"left_control"
]
}
],
"type": "basic"
}
]
}

This achieves what I want but works erratically: the modified key input sometimes outputs \ and sometimes ç (which is what's in the "backslash" position in the Spanish keyboard, not in the Polish keyboard).

Is there any way to fix it? Maybe by introducing a little delay between the key_codes in the "to" section? I tried drawing inspiration from pqrs-org/Karabiner-Elements#2664 but couldn't get any working code :( Thank you!