Crack in aluminium frame near dropouts

You may want to think this through more carefully.

Carbon can be easily stiffer than aluminium, and therefore though potentially stronger, it is even more expensive/difficult to repair should a crack develop.

Good luck with that. There isn't an Al frame on the face of the planet that's worth repairing, full stop. If it's cracked, it's fooked, simple as. Carbon frames can be repaired relatively easily and cheaply, depending on the damage, including as cheaply as a bit of body filler. And with a carbon frame, you can ignore some cracks entirely.