It's impossible to tell if there was any long-term plan for Carly and Ben as a couple. My guess is that they signed Julian McMahon up because he was a good-looking young man and they always like good-looking people on Home and Away. They also needed to do something with Carly. If Julian hadn't decided to leave so quickly, Ben would've become a love interest for other "lucky" ladies in the Bay. I can't see how that marriage to Carly would've lasted if they hadn't moved away. Sharyn probably would've left, no matter what else was going on. And much as I like Carly, she's not someone I could see as a foster parent in the early 1990s. I wouldn't inflict Ben on them either, but that's another story. Although the show had lost Tom, I don't think they were planning on sending foster kids to anybody other than the established older adults. They'd already expanded the fosterverse in Summer Bay to include Fisher and the Stewarts. Perhaps there's something to be read into Bobby becoming a foster parent after they'd steadied the ship at the caravan park.
I don't have any strong feelings about Carly and Ben's timeline for leaving. Having said that, his reputation would've improved if they had sent them off into the sunset straight away. There were times when Ben was either downright awful, or an idiot. That foray into farming is a bit cringey at times, even though it's clearly a build-up to him re-enlisting. Carly, too, would've suffered. As has been mentioned many times here, the early 1990's wasn't a good time for young adult characters. They'd painted themselves into a corner with Carly. She hated working in the diner, she wouldn't be going on to further education, and she'd aged out of being a foster kid. It's better that she and Ben moved on,