I’d say Worcester and Lowell both sufficiently fit your description. Swiftly Developing cities with pretty vibrant downtowns, a commuter rail line straight into Boston, and still relatively affordable (though it’s getting a lot worse especially for a single income). The kicker is that they’re both still a bit gritty. If you haven’t been to former industrial towns in New England, you’ll understand when you get here. But don’t let that out you off. They’re safe.

My personal pick would be Worcester. 2nd largest city by population in New England and it’s growing very fast. Lots of food and bars and shopping and what-not. Also lots of parks and nature.