1. he seems like the type to be really good at painting/calligraphy/other artistic tasks that require a steady hand/eye for fine details. he has an eye for color, depth, etc.!
2. Sukuna gets bored with things quickly but when he cares about something he has a lot of patience. he's very diligent and will spend a lot of time perfecting things/honing his talents
3. he either LOVED riding horses, or horses hated him and he couldn't ride them, no in between. he seems like the type who would have been into horse racing tho, so I'd say it was the former.
4. I don't think we need a backstory for him, he serves his thematic purpose well, but I'd like it personally if we could learn more about his mom. I like the starvation/famine headcanons for his childhood & upbringing, with him being from a poor family rather than a big clan. he comes across as a very self-made man to me, maybe born to non-sorcerers? I like the idea of him being rejected by his parents too, or maybe rejected by his father but accepted by his mother.

these are just a few off the top of my head!! if there's anything specific you want my thoughts on feel free to send another ask!! I like talking about Sukuna