Why is 100mm too thin? And aerated blocks are frost rated

I would go for a 3.5N 100mm solid block for this type of wall.

Hollow blocks can crack under the slightest movement, and a 7N block
would be a pain to lay. Also 140mm is overkill and way too heavy for
this application.

Don't forget to build a peir at each end of the wall for stability.

I also like the way you have thought about the aesthetics - covered
from your side and a nice grey mass from the other side. Don't forget
to leave all the mortar snots hanging on that side too, just for
completeness

dg