I thought it was time travel, but it was actually a system that the male protagonist, a scientist, spent nearly a hundred years building in order to meet the female protagonist.
The male protagonist had a crush on the female protagonist in modern times for 3 years, and later she died at the age of 30
After the reincarnation, the heroine returned to 16 years old, had healthy parents, and even silently did a lot of things to protect her. Then she met the hero, got entangled, grew up, and understood love, family affection, and friendship.
