Suggestions:

Decoupling elevation from geopolitical boundaries may allow more of the detail to show through, if that's desirable. One colour per country is quite a coarse quantisation.

Try generating per-pixel rather than per-country average elevations and use a carefully chosen contrast colour to draw national boundaries.

Perhaps consider switching from the current blue-white-red colour mapping to an alternative with a couple more hues, continuing to account for people with colour perception differences.

Compressing the blue-white range as you have for differentiating commonly-occurring low elevations does add some visual clarity. However there's a tradeoff with the linear consistency of the scale (outcome: distinguishing mid-range and higher elevations is harder).

The maps and atlases in your local public / school / college library (or other cartography resources online) may be helpful.

Here's a classic legend generator: ColorBrewer: Color Advice for Maps

Suggestions:

The thick, red, high-contrast lines separating the bars are reducing readability and clarity.

Aligning the flags at the right-hand end of the X axis labels would allow groupings to stand out more.

Swapping the X and Y axes would let you use larger text for the embedded revenue values and orient the smallest text more comfortably. You'd lose the fuel-filled vessel metaphor, but I'm not sure that's currently adding a lot.