Start with the branch that fixes travel or fill.
Character upgrades are a strong default when movement is the slowest part, because every lake-to-drain trip benefits from faster travel. That does not replace bucket upgrades when fill speed or capacity is the real blocker.
- Travel bottleneck: Character or movement speed first.
- Fill bottleneck: bucket fill speed or collection efficiency.
- Capacity bottleneck: bucket capacity before deeper routes.
- Depth bottleneck: checkpoint progress before risky long pushes.