How to progress with separation training?

I adopted a lurcher a month ago, abandoned by previous family, whom I picked up from the shelter. He was already neutered and is thought to be about 5 years old.

He’s been doing very well inside the house, and very good with suburban city noises. Not good with animals and other dogs. I’m on a 3 month sabbatical so have 2 left to go. He’s pretty independent inside the home, always preferring to sleep away from me but follows me around when I’m doing something interesting or when he’s bored. Sometimes I forget he exists… I’ve started desensitising him to the door and he can manage a few minutes alone when I need to throw out trash or fetch the mail. My neighbour has been kind to pop in when I need to run errands but sadly she’s moving next month and the new neighbour doesn’t take kindly to dogs…

After I leave, it takes him a few seconds to come to the door and whines. The whining is sometimes only a few seconds or it can be on and off, but he always ends up lying down one way or another. I’ve not needed to test the waters beyond a few minutes so I have no idea what he will do. How do I know he’s ready for anything longer? I have cameras set up already. Like many lurchers he’s quite vocal during the day. I keep hearing people comment on the multitude of Reddit posts on this saying to never leave them longer they can tolerate but I don’t know what that is. My sister comes to the city once a week for work and drops by for dinner and he whines when she leaves, so I think it’s more nuanced than just blanket separation anxiety. He doesn’t look 100% relaxed if I leave though.

I’d like him to manage 5 hours. I can have a walker come by during the work day but when during the day isn’t guaranteed yet. Does his progress sound promising so far?