Addressing separation anxiety takes time.
For your dog's safety, I suggest addressing a better crate first. One thing you may want to try is a crate like this. I’ve never used this crate, so I don’t know how durable it is, but because of how it’s designed it’s harder to chew on. You may want to reinforce the lock with two hasps one on each the top and bottom. Or four hasps, top and bottom on the left and right sides of the door, I don’t know how strong the crate is. Just make sure to secure any hasps in a way that screws don’t go through the door, one option is Liquid Nails which may be combined with small screws that won’t go through the door, but Liquid Nails should be sufficient if applied correctly. You may also want to put this crate in a corner where it can have strong walls on as many sides as possible.
There are more secure crates, ones made of all metal with solid walls, however those tend to be very expensive.
The most important things (besides a physically safe environment) are
The dog always knows that you will come back
Leave for less than a minute, and then come back, but don’t make a big deal of it. Just act like you are leaving for a while (jacket, keys, etc) and then go outside, wait, and come back in. Do this a few times a day. You don't need to put the dog in the crate for this.
Once you can leave for a minute, make it two, then five. Once you are at five minutes, put the dog in the crate, then get your stuff, then leave.
When you come back, let the dog out of the crate but don't make a big deal. Let him out, go get some water, sit down, act normal.
The key is that the dog knows you are coming back, and that this is normal activity.
The crate is a fun, and low-stress place for your dog
Fill Kongs with your dog's highest-value treats and put them in the crate whenever you leave. You want to build a positive association with the crate, and the easiest way to do this is with food.
Try covering the crate with blankets, you can get moving blanketscheap on Amazon.
Or for a little more you can get something a little nicer if you don’t want to look at an ugly moving blanket.
I suggest getting the 12 pack, dogs sometimes destroy the crate covers so it's handy to have spares.
The idea is just to make the crate dark and to help it be a calm, distraction-free environment. Whenever you put the dog in the crate, cover it completely with the blanket. It won't block sound, but keeping it dark helps.
I don't know if this product works, I'm not affiliated with the company and I've never used it before but try a Snuggle Puppy.
AKC calls it: >This is the ideal toy for anxious puppies.
Just note that AKC's description is on a page trying to sell you the product, so be cautious.
I also suggest a conversation with your vet about anxiety meds, something that is acts quickly and doesn’t stay in the dog’s system too long. Meds are not a solution, but they can help. If the dog is so anxious he can’t focus on the Kong, it doesn’t matter how good the Kong is. The right medical solution helps the dog focus on the training. I’ve used anxiety medicine with dogs with good results.
Exercise
Make sure your dog is getting enough exercise, a tired dog is a calm dog. It’s hard to be anxious when passed out from playing and walks.
Try anything
You said you are willing to try anything, so here are some solutions that are less than proven.