Living alone is not the same as social isolation. The way you describe your life is sounds obvious you are not socially isolated or socially disconnected. I am sorry but I don't feel equipped to explain it in full but there is plenty information on the internet. You came into money and were able to buy your dream home in a place you chose. You had the resources to move across the country. Social isolation is more about lack of resources that creates depression and isolation. I am in a place I moved to because of spouse's job. He divorced me, remarried, and moved away, and I stayed to raise our child to adulthood and due to lack of resources got stuck here. This is not a place I would have ever moved to on my own.

I did note living alone as contributing to my social isolation. If I had the resources I would move to be near my adult child, his family, and my extended family. Because I don't own a home I may choose a housemates situation with residents of different ages. My single brother has lived that way his entire adult life and he loves it. He has lived with all ages and genders. We came from a very large Italian Irish family so a full house is comfortable for us.

If I won the lottery I would consider the purchase of a vintage townhouse in a large colonial city on the East Coast. I would spend my days in museums, libraries, at the theater, and in cute cafes. I would live near a park. Weekends I would take a train to the country and seaside. My perfect life. No pets. But yes to a cottage garden.