Desperate not succeed gradual leaving after 2 months

Hi everyone, I really need some advice because I’m feeling quite overwhelmed of his barking when he is alone at home 😢.

I tried gradual leaving 10 minutes to 40 minutes with/ without shoes/keys and it isworking fine . However when I have gone downstairs and I am really leaving, he starts barking about 10 minutes later 🥺.

Even we did the exercise of leaving him one room upto 1 hour is working fine.

So it’s not instant panic, but it builds up after a few minutes.I’m worried about his stress level and also about disturbing neighbors.

Should I reduce the duration again?

How do I stop the barking once it starts after 10 minutes?

I would really appreciate practical advice. Thank you 🙏