Wow that sounds interesting! Enjoy this time.
I could choose the boots, so actually I do not really know how to suggest that. But I can give my impression.
I do have a BZ400x, very happy but as I said no comparisson. 6°C was the coldest, however just say about 20min, I had with my drysuit. It did at most feel a little fresh, but really very ok. And i just had a thin base layer. Using additonal merino middle layer and a thin base layer would make it even better.
I wear the Santi Primaloft socks. I did not feel the cold at all, it felt warm! However they need big shoes.
I need to push my feet into the boots a little bit, when I have them plus some thinner base layer socks. Then they are good fitting, so it does not feel uncomfortable or some pressing anywhere.
If I just use a thick sock instead of the primaloft (say for summer) my feet wobble a bit, so would move by about 0.5 cm if i go down a hill.
Having said this, I again want to stress I have it new and no comparisson.
In my semi-dry with a vest I did shiver at these conditions, especially the feet have been cold.