4 months of trying and puppy still won’t sleep alone. Is it too late for the crying it out method?

It is 2 am as I'm writing this at my wits end. Ever since I brought home my puppy, almost 4 months ago now, I have been gradually working towards him sleeping alone downstairs in his crate.

The first week, I had his crate close to the couch, where he could see me and I could touch the crate to calm him. The weeks after, I gradually moved his crate back, teaching him to be comfortable with more distance. This went well and after some tries, he finally slept through the night alone after a month. For about 3 weeks he slept alone in his crate from about 11pm to 6am and sometimes even some more after potty break.

Then he started to get some regression around the time he started teething. Waking up to potty earlier and earlier and no more settling down after. So I started spending half nights on the couch again. But over the next weeks, he went downhill even more, and my presence on the couch was not enough to settle down anymore, he needed to be closer again. I cried many nights over my failures, then sucked it up and started sleep training from scratch. But instead of moving the crate, I moved myself this time using our one person sofa bed.

I started very close to the crate again and once he was sleeping through the night, I created more distance. I repeated this process several times and every step came with several bouts of crying, which decreased over nights, until he slept through the night again. Tonight is the first night he can't see me anymore. He has slept for 1 hour and now has been whining / barking for 2+ hours. I tried to calm him from a distance, from close by, have gotten angry and have now been trying to ignore him for 1 hour. The only thing that will calm him, is open the door, which I refuse. I just don't see how he'll ever sleep alone again. So now I'm considering letting him cry it out for a few nights as I really don't see what else I can do anymore. He is 6 mo now and I really need the sleep to deal with puppy adolescence during the day. I am borderline burnt out because of the 4 month-long sleep deprivation.

Some extra info: his crate is covered with a blanket, he has bedding in there that he likes, he walks in and out of his crate during the day without issues, we never leave him alone in a crate during the day, only in his pen, to avoid negative experiences, I leave a tiny bit of water inside over night, he gets sufficient mental and physical stimulation, so he is knocked out 60/30min before bed, and his health has recently been checked by a vet.