Only the flight school was mandatory. I would always not do missions I didn't like unless I had to, at least when I was a teen. That's how I also found out Zero's missions weren't mandatory.
Well, I can't speak for your experiences in playing a game. I thought San Andreas was a laidback experience, and I still do while acknowledging its many flaws. I'm probably just partial to it because of nostalgia. And I've never played VCS. I have LCS, but I never finished it.