She played the captain of the Lakota later in DS9, Captain Benteen.

The first time they introduced the Eugenics Wars set in the 1990s, Spock called it the last world war. Pike in SNW elaborated that it was part of the escalating conflict that went nuclear as WW3 decades later.

I’d say TNG mostly stopped exploring new frontiers halfway through season 1. Farpoint promised exploration, but soon the ship is ferrying diplomats and scientists and answering Federation distress calls. The worlds are new to the audience, but not the characters.

TNG also had Vulcan extremists (trying to recover ancient psyonic weapons) and Sela must have expected a fifth column ready to defect once her thousand troops landed and gave them an excuse.

I’ve always thought the Romulans weren’t just the Vulcans who rejected Surak’s teachings, but also any who didn’t have the physical ability to follow them.

Originally it was just based on Romulans not expressing any psychic abilities, but Picard also established that sharing personal details publicly as a major taboo. That would track with them being a mix of former enemies who are concerned with suppressing the rivalries that lead to nuclear war.