How is paint embrittlement countered in high altitude aircraft?
There are 3 critical areas one may wish to address with paint:  the formulation of the paint, how it is attached to the substrate, and the properties of the substrate.

The substrate may expand, contract, bend or twist with heating, cooling, or stress.

The attachment of the paint to the substrate is crucial to durability.  Surface preparation includes cleaning, deposition of a chemical coating such as chromate, or primer.  This is a make or break step that should not be ignored.

Finally the paint itself.  Bare bones paint formula is solvent, pigment, and resin. The solvent evaporates, leaving the pigment (color or shade) and the resin, which is essentially the coating.

Factors such as temperature, flexability, hardness, weather and UV resistance must be considered.  The closer the properties of the paint are to the substrate the better.

I would find out what the airliners use.