Separation Anxiety?

For the last several days Ivan has pooped in the house. He was always really good at letting me know when he needs to go outside.

I am starting to be convinced that he's doing it out of frustration. I give him as much attention as possible considering his issues and all but I can't give him attention 24-7 since I also have my own dogs to take care of. My dogs also need some time with me to either hang out or play outside. Don't think I leave Ivan alone all day, I take him out for a lot of walks since I don't have a fenced yard and I don't trust him to stick around of let loose and I also hang out with him in the kitchen, then alternate my time between him and my other dogs. When i'm not with him, he's becoming more vocal and causing a fuss. It seems like he wants to trust me and approaches me more often but it's usually short lived. I want to build trust with him but I don't want to create SE in him either.

This whole behaviourist/trainer thing is taking forever. It takes them so long to get back to me with things and everything has to be approved by the rescue before bringing in anyone to help me with Ivan.

I am home all day, so it's not like I only have a short time with the dogs before turning in for the night. I only leave the house once a week for about 2.5 hours. The rest of the week, i'm home.

This sounds like more of a house training issue than a sep anx issue. Are you taking him out leashed and making sure he is emptying his bowels before being in the house? Reward him by interacting, running and playing or even treats after he goes outside. Take him to the same location each time so he learns what you want him to do. You could also put it cue. (capture the behavior when he is doing it). Timing structure is key. Feed the same time each day (or twice a day).
Can you confine him in a small room or Xpen or crate when you can't supervise him in the house?

Good luck.