aah thank you so much!! I completely get that being difficult! and I can tell you I definitely didn't start with their backstories. Every writer is different of course, so I can only tell you what works for me. You'll have to take what resonates and leave the rest! I didn't start with their backstories; in fact, when I started writing, I knew as little as Felix did. Everything (and everyone) slowly took shape as I continued to write. It definitely helped that Felix didn't meet them all at once, so I didn't have to introduce them and their jobs/studies all in one go. I think I just picked things that I felt made sense for their characters at the time. They're not even the most original jobs; we see them for these 8 men in plenty of other fanfics. But they felt right for the silly little story I was writing for myself at the time, and then it grew from there. Sometimes their jobs were just a fun choice (like Hyunjin being a painter) and I explored how it would fit into his backstory later, and sometimes their jobs/studies were plot-based (like Seungmin studying medicine). I did like the idea of him being a student though, of him not having all the answers on his own yet, and so that's what I wrote.

I'm not sure I'm giving you any answer at all lol, but I hope some of it is useful anyway! I guess the long and short of it is to just pick something that seems fun to write, or fun to explore when it comes to their backgrounds or your plot. And if you have an epiphany 4 chapters later and realise that one of them should really be studying X instead of Y, you can go back and change it! It's your story, just have fun writing it! <3