I like this a lot~

Isekai but with a terminally ill mc who transmigrates as a child who will die when they hit the age 21. I really like this because she's actually enjoying living life as a "normal" heathy child, running around enjoying and appreciating everything she couldn't as a sick patient. She's not worried about dying at 21 either because being able to live this second life as a healthy child is more than enough, so instead of trying to avoid her death, she's just making sure her family lives.