Evasion cannot be used against Disintegrate - correct? Because Disintegrate is not a save for half damage spell - it's all or nothing.

Evasion
[...] When you are subjected to an effect that allows you to make a Dexterity saving throw to take only half damage, you instead take no damage if you succeed on the saving throw, and only half damage if you fail.

Correct.  It turns save for half spells into save or suck spells, but does nothing for spells that are already save or suck.  Disintegrate is a save or suck spell already, so Evasion does nothing

Yep