OP - your current boss sounds a bit like my line manager, except for he does at least respond to emails. He's a decent enough bloke and we normally get on quite well, but he's not half as switched on as my previous line manager, who was a woman, and had much more empathy than him.
From what you say, it's not the actual work that's causing you problems, it's the isolation and lack of human interaction with colleagues. I totally get this, but I'm not sure that going off sick will help you, because you'd have even more time to ponder, in isolation.
I used to have quite a laugh with my line manager, pre Covid. If we bumped into each other we'd have a little chat and sometimes it felt like we were quite good friends. Now it's very different, he's under immense pressure, working long hours while others are on furlough, and I know that he doesn't have the time or the patience with small talk that he once had. He's been a bit prickly with me at times, during our redundancy consultation. It hurts, but I have to remember he's not my friend, his job isn't to make me feel better about things, and during this period, he's only interested in handling my redundancy by the book so that I have no cause to challenge it in a tribunal.
I'm glad you started this thread, where people in similar situations can compare notes and have a virtual chat. It's pourimg with rain here, but I'm off for my long daily walk soon, when the rain is expected to clear. One plus to furlough is that I've lengthened my walks, which has resulted in losing a few pounds.
That's another reason I force myself to walk every day, even if I get soaked.