I think it helps to live in a city. The cities have their problems, they're expensive and dirty, but there are loads of older single adults, and even an introvert can sit at the same bar once a week and meet people, or go to meetups or a weekly class and slowly meet people. Hell, in bad times even chatting with a bartender was nice.

Being in your shoes in a suburban setting surrounded by families and neat little houses (or huge fields in a country setting) I imagine is hard. I'm only making assumptions.

Have you thought about traveling long weekends or abroad longer? Perfectly doable solo.

Meetups aren't as effective as you think. I have seen complaints from multiple large European cities that general social meetups don't actually result much in people exchanging contacts and meeting again in future; it feels like a one-off encounter with other human beings, and that makes it dismal. There is even a predatory industry now of for-profit social meetups because people are so lonely, and knowing that one is being exploited only makes it more dismal.

For more specialized hobby meetups, it is a common complaint that one's fellows are only met while practicing the hobby, they never become general friends that one can meet regularly with, let alone turn to in emotionally heavy times.

I wouldn't recommend solo travel as a way to socialize unless one knows the person's age. It's a sad fact that over the age of about 30-35, solo travelers (especially men) can no longer take advantage of the social opportunities of the backpacker scene; you might get a few nice chats, but most travelers will be very wary of you.