One additional technique that helps with spaghetti plots at this scale is to overlay a smoothed group trend (for example, LOESS or spline by income group) on top of the individual country lines. That keeps the individual trajectories visible while giving a statistically clearer cohort signal. You can also pre-cluster countries by trajectory shape and panel by cluster instead of income group to reveal pattern families rather than just economic tiers. This often surfaces structural differences that color grouping alone misses.