If I were to cast True Polymorph (which permits all creatures, not just beasts, as the regular Polymorph does) on myself to turn myself into, say, an Ancient Brass Dragon (provided that I'm level 20, which is required due that dragon's CR) - could I then use the Change Shape "racial" feature of the dragon?
Change Shape. The dragon magically polymorphs into a humanoid or beast that has a challenge rating no higher than its own, or back into its true form...
Choose one creature [...] that you can see within range. You transform the creature into a different creature, [...]. The transformation lasts for the duration, or until the target drops to 0 hit points or dies. If you concentrate on this spell for the full duration, the transformation becomes permanent.
Shapechangers aren’t affected by this spell. [...].
Creature into Creature
[...] The target’s game statistics, including mental ability scores, are replaced by the statistics of the new form. It retains its alignment and personality. [...]
I can't find anything in the spell that would prevent one from doing so. The only thing that might come into play is the phrase "Shapechangers aren't affected by this spell". You as the caster are not a shapechanger, however, so even though you transform into a shapechanger, you aren't when you cast the spell.