I had to recently have a random eval for disability. I was outrageously anxious before-hand, especially because the letter had me seeing a male evaluator (causes me huge anxiety), but it wasn't that bad. I ended up talking to a female psychologist (I had also initially been told it was a psychiatric assessment, not psychological). The whole thing took about 30 minutes. She asked a bunch of history questions, and covered the current stuff a bit also. None of it went into too much detail, so none of it was very triggering for me.
My eval seemed very random to me too, because I currently see a therapist regularly. My worker said she wanted it because I had been to the hospital recently without a psychiatric follow-up (the resources the hospital gave me had a 2-month waiting list). I had also gotten letters stating that various providers had not sent in my records, and I needed to follow-up with them to see if I could get it faster. I have a lot of anxiety talking on the phone to strangers, so it freaked me out to have to make 4 or 5 phone calls. I finally called them, only to find they had sent the records over a week ago. When I called my worker, she said that she had gotten all the records after she sent me the letters. She confirmed that they will often send out letters preemptively to make sure they get everything.
I would suggest trying to lessen the anxiety and go through with the assessment if you can. Any extra evidence you have stating you need the disability will help them decide on things. Also, I would suggest trying to call your worker again and making a connection. It helps a lot if your worker is also in favor or you getting disability... I know it's hard to make that call, especially since your worker sounds a little frustrated with the job, but maybe you can call and find a reason to have a conversation?
Good luck!!