BC puppy and separation training without crate

I have a 5-month-old border collie puppy and up until now he has always been with someone – either a sitter, at my grandmother’s countryside home or with me while I work from home. He is quite attached to me and follows me most of the time, coming with me to another room about 8 times out of 10 and always watching what I’m doing.

I haven’t done intensive crate training because he seems more comfortable moving freely around the apartment and is able to calm down and fall asleep either in his bed or on the couch.

Now the time has come to start training him more seriously for being alone. So far, I’ve been using a separate, puppy-proofed room in the apartment. I leave him there with a Kong and close the door behind me, usually for about 3–10 minutes. I probably couldn’t even leave for 15 minutes, because as soon as I open the front door, he starts barking and tries to open the door.

How can I effectively train alone time without using a crate? Has anyone successfully done this? I know that I have to get him exhausted to leave him alone for even a couple of minutes but what's other tricks to train this efficiently?

Also, how much did your bc puppy sleep during puppyhood? Mine sleeps no more than 14-15 hours per 24h.

Thanks in advance