Struggling with carrier training for airplane

I have a toy australian shepherd, 16 lbs, who I've been trying to train to be comfortable with his carrier. It's for a relocation to Japan so it's really important that we can get it down. We still have about 6-8 months before relocating but I have been working for days with him and feeling very defeated.

He is not crate trained, and he is very smart and anxious so he's hard to train. He will go in to get treats and lick the lick mat, but is very anxious the whole time and if I touch him or even say anything, even "good boy", he gets nervous and wants to exit, and usually just straight up backs out. Sometimes he won't even go in to get the treats out of fear.

I've been doing this:

– Throw treat in, sometimes tell him to sit and stay, and then release, tell him "crate" and point in towards the treat.

– He goes in, grabs the treat, goes out immediately. He keeps his feet outside the carrier and leans in so he doesn't have to be all the way in.

– I repeat the process, sometimes breaking the treat apart and sprinkling it inside so he has to lick up and spend more time in there

– Sometimes I cover a lick mat with buddy butter, crushed up treats, and put it in, and sometimes he'll spend more time licking it but other times be jumpy and only go in for a few seconds before coming out.

– A few times I've gotten him all the way in with food distracting him inside, and I did manage to close the carrier even though he freaked out a bit. Then I told him to stay as I opened the carrier, made him stay inside with the carrier open for a few seconds, and released him on command with lots of treats. But there isn't really any progress.

I'm trying to work my way to be able to close him in the carrier, and get him comfortable turning around inside, and eventually laying down and relaxing in there.

At this point it's super hard to imagine him being able to sit under the seat in front of me in his carrier on a plane for 12 hours. I will definitely be giving him trazodone for the flight but I still can't see how we'll ever get there when he can barely feel comfortable going into the carrier at all.

Any advice? People with aussies? People who have flown with them or other stubborn dogs in cabin in a carrier? Any advice for relocation? Thank you.