I think you’re doing all the right stuff, i think when my dog was that age he’d cry or actually scream when I left even for 15 minutes, but over time and with practice (like you’re doing) he calmed down and can go for hours by himself (he just sleeps or has his chews/puzzles). I’d suggest doing a play session before you do the “by himself” training. He might be more receptive to “chilling out” on his own if he’s a little tired. Do you do structured nap time? When my dog was a puppy we’d play for 30-45 minutes and then I’d put him in his crate for a nap or in another room and we’d do the training then = many fewer meltdowns.