I don't think Lammy was a disaster as foreign sec, though he was good for some memes.
The idea of Lammy as our top diplomat seems funny on the surface, but on a personal level he's naturally easygoing and makes friends easily. The weird odd-couple friendship he's struck up with Vance is a case in point. Those bits of the job where doing your homework was less important, and relationship building more important, he was quite good at.
I suspect with Cooper it will be the opposite.
Shabana as home sec is a no-brainer. She's tough and smart, she's the closest thing Labour have to a law and order and immigration hardliner, and being an ethnic minority woman makes her somewhat fireproof.