By this, do you mean the private active key of the witness account, or the same one generated in the previous step, when we did "suggest_brain_key"?

You'll want to import your private active key first to be able to broadcast the operation to enable your witness, I also recommend importing the key from suggest_brain_key as some CLI commands use it(witness_update actions I believe is what they are called).

The witness already has more than 24 hours synchronized and printing the lines that I indicate in the image

That's normal. It's just how your node is spending most of its, time, it's mostly idle, which is good since it's not getting overloaded.

Do you think everything is fine?

As long as your key is correct in the config.ini(most common issue I've seen people do), you are good to go. Now go out there and get more votes, best of luck to you.