Yeah those all sound nice in theory but those activities way more likely to be populated with other weird lonely overthinker types like OP. Do stuff that normal people actually do, not what internet people think normal people do.
I would be highly skeptical that most people in their 30s doing ballroom dancing and bird watching are normal. I know you're just making stuff up with the D&D inclusion.
Those were just example. Any big city is bound to have thousands of groups like these and you'll have to find one that fits. And even being with other lonely nerds can be stimulating enough to blunt depression enough to get you moving. If you just assume everything in the world sucks and to not even try then that's how you get lonely.
I don't know about bird-watching, but ballroom dancing is really big in computer science. It was really big in my CS department on the east coast 20 years ago, and it's really big at Stanford now.
As far as I understand it, software engineers in the Bay Area have two common hobbies: indoor rock-climbing at Planet Granite and ballroom dancing. I would never have guessed how common they are, but it is what it is. D&D is a distant third.