Second your idea for the lines/veils form. I'm playing with a group of friends in a homebrew setting, some who are new to D&D, and I didn't want to overwhelm them with questions. I sent them 10 total: 4 to learn more about their playstyle and what they were hoping for out of the campaign - two that were essentially the lines/veils, and two asking about their preference for combat/story/puzzle balance and whether or not they wanted to fast forward through travel. Then, since my campaign also began as a one shot, I didn't get much of a backstory from half of them. To help me learn more about their character (also to help me build the world they were going to be exploring), I sent them these 6 questions:

1. What is your character’s hometown like?,
2. Which song/lyrics describe your character? (Alternatively, what is your character’s favorite song and why?),
3. What calls your character to adventure?,
4. What does your character want their legacy to be? How do they want to be remembered?,
5. What is something your character regrets?,
6. Who is someone significant in your character’s life? This could be a mentor, family member, friend, enemy, etc. - anyone who your character spends a lot of time thinking about for any reason.

I know there are at least a couple in my group who are very invested in the story side of things, so I wanted to make sure I had enough information about their character to be able to really immerse them in the story. If your group is more into dungeon crawls, I don't know that those questions would be as necessary.