Rescue dog: Strange behavior with food/general anxiety

Hey all. This is my first post on here, and thanks in advance for any thoughts. Apologies for how long it got.

I have a senior rescue pup that I adopted in October 2020. I named him Munchkin, we think he’s a yorkiepoo, and most vets estimate him to be about 8-10 years old.
I adopted him when my husband and I still lived in Delaware (I’m from PA). He was found on the streets of Philadelphia, extremely matted, and brown when he should have been black. I scooped him REAL fast when the rescue posted him, and it was love at first sight. He’s always been fully house trained and I groom him (shave/bathe/etc) for several hours every four weeks, and he’s just got a huge and adorable personality. We now live in Oklahoma, so there have been a lot of changes in our lives, but we still love him just the same. He likes it a lot better here too.

Now that we’ve covered some backstory, I’m here about some really odd behaviors he has always done. I’m just trying to understand him so that I can help him.

His biggest issue is separation anxiety. I have read some things on that already, but I’m still going to elaborate on what Munchkin does.
It has gotten worse over time, but he barks incessantly if we leave. He’ll pull cushions or blankets off the couch or bed and throw them around the house but not tear them up. He’ll get into any food he can. He’ll find a way into trash cans or the litter box. He also often stares at me very, very intently at all times without moving, and will sometimes look away immediately if I look at him. I’ll do a playful voice and pet him when that happens most times, or I just let him be. It wasn’t always this way; we used to let him free roam with little issues and no barking.
Needless to say we have had to start crating him for his own safety. He doesn’t understand toys so that doesn’t work, but I do have a Kong and a squeaky/soft mallard duck for him. The Kong works until it’s empty then he starts barking. If I have a blanket in the cage he rips it up, but he specifically only rips up blankets. Lately, the anxiety has gotten so bad that when he notices leaving cues, he starts panting and shaking. I truly don’t understand why he fears being crated; we just tell him to go lay down about 5-10 minutes before we leave, and give him a treat when he goes in. We make no fuss when we leave or come home either.

A connected issue is insecurity around food. He genuinely seems to believe that if he has food in his bowl, he will never ever have it again. He has a huge issue with moving it around with his nose and stuffing it under anything nearby, or just tips it altogether. He does this with wet food as well. If I put him in a crate with food, he immediately tips the whole bowl and moves it around until it’s all entirely out of the crate, defeating the purpose of giving him food. Same with water. If I pick his bowl up or move it to mop or vacuum or what have you, he usually either shakes or starts cowering or hides the food. Meanwhile I’m just kinda standing there watching like… not sure what to do. If I positively interact, he keeps acting very anxious anyway until I set the bowl back where it was, and then he’ll go over and thoroughly inspect it but not eat. Sometimes he’ll start hiding it. I have never revoked food so I’m very concerned on this one.

Those are his two biggest issues. Another one is pretty bad dog insecurity. He’s very nervous around other dogs but not hostile. He does play with certain dogs but it’s pretty random when he chooses to bark nervously or chooses to play peacefully or chooses to run away.

There’s really only a few things I can think of that could have made this worse, but I’m not sure how to reverse the effects of those specific things.

One thing is that we’ve had a few other dogs pass through. We temporarily had the dog of my husband’s bio father because the father was passing away and the family could not care for the puppy. She was a rambunctious Belgian Malinois. We moved in with the rest of the family here in OK last July after the father’s death and returned the dog to them. (We had the dog April to July.) All was well except that, when we left the house and HAD to leave Munchkin behind, they would let Munchkin out of our designated bedroom against our wishes to make him stop barking. The other dog would proceed to play too rough with him and they did little to nothing to stop it. We were furious and we don’t speak to them anymore for many reasons. We also brought Munchkin with us no matter what after it happened.

The other two dogs weren't planned, but hopefully I can explain.
Back in February I decided to get him a crate and I got it off of Facebook marketplace. The people I got it from were severely abusing their dog. I worked with the local police and I ended up housing the dog for a night, and then drove him about two hours away to a rescue the next day. He’s okay now and got adopted, but I understand if this had upset Munchkin despite them being separated and the other dog being very sweet.
Similarly, someone dumped a pregnant dog at our apartment complex in late March. Munchkin actually really liked her, but I had her for two weeks before she was transported to Chicago to be fostered and have her puppies. That dog unfortunately despised cats and our own kitty came first, so I showed the dog a lot of love (and Munchkin simultaneously) while I had her and then sent her on her way in early April. Once again, I see how that could affect him. But there are very, very little resources in OK for animals. These two dogs would have been euthanized if I did not do what I did to help them.

The final thing is that my husband’s sister has a very intense special needs toddler. They live far away so we sometimes had to take Munchkin, who we kept totally separate—but I’m sure the noise and tantrums still upset him. Last time I wasn’t fast enough to hide him and the kid unexpectedly yelled in his face. I think this one affected him the most but I’m not sure. I regret it deeply and don’t take him there under any circumstances anymore.

This whole post kind of focuses on the bad stuff, but he overall has a very quiet and fun life. I’m going to try some calming supplements soon, but I really could use some help better understanding his behaviors. (Or at least validation for what I’m already thinking.) I think this stems from a heartbreaking abandonment situation or similar and I hope I can help him believe that neither him nor me are going anywhere.

Thanks y’all. I’m just trying my best out here.