I am sorry to bang on, but, having found and ticked a ‘unify Sent items’ option, I cannot actually find a way of viewing that unified folder (thingy).
On the screen where you can select the folder to be displayed for an account above Inbox you should see Unified Inbox
Ah: enabling the Unified setting shows sent messages (from all accounts?) in the . . inbox. I submit that this is counter-intuitive and pointless. Why not offer an actual unified sent items?
That would be a feature request I guess. At the moment the option is “Show Unified Inbox”. And that is what you get.
Do you see an option “Show Unified Sent Items”?