Okay, I've solved it.
As unbelievable as it may seem, I read the documentation dozens of times, but I kept mixing up the names of the variables between:
"Example credentials file using restricted API Token (recommended)"
and
"Example credentials file using Global API Key (not recommended)"
So, I was creating an API TOKEN type key and saving it in the file as dns_cloudflare_api_key.
It's unbelievable, I kept re-reading everything and nothing was working.
I realized the mistake on the hundredth read, almost by accident... and now it works!!!
Thx to everybody for the help!!! 