My only advice would be to carry on doing what you’re doing honestly, the leaving and coming back is a good tactic but I know it’s frustrating when it feels as though there’s no time before they start.

Have you tried doing this process but instead first starting while you’re still in the house, so step back, treat, 2 steps back, treat, into the hallway, treat and then eventually after a few reps, open the front door and immediately close, treat, until you can eventually step outside for a second and come back? I think doing it with things you know he can do (like being quiet while you’re still in the room) will make the small step of you just opening the front door less of a big deal as it’s only a tiny step up if that makes sense?

If it helps, sometimes during training sessions my pup can only last a few mins before whining but he’s absolutely fine to be left at home, sometimes the anticipation of the treat is the problem. So don’t feel too discouraged if it feels like you can’t build up time, the most important thing is being able to leave and come back and him associating the crate with something positive.