Thanks a lot, Leica seems to be the winner indeed (and the zoom range is more interesting).
I'm still quite stuck between two options here for my budget :
Option 1 : Ikelite housing with 8" Dome port + Leica 8-18 with extension and zoom ring (which is the same as the 12-60 I already own). So that's a pretty polyvalent and versatile option
Option 2 : Nauticam housing with 7" acrylic dome with zoom ring for the 12-60 + vacuum.
With these options I will have 2 Weefine Solar flare 3600 lumens (I think it's the same brand as Kraken)
These 2 options are about the same price but I got more flexibility with Ikelite from the start but also bulkiness and less upgrade options (like an external monitor...).
The Nauticam one is a more restricted option (for now) since I could only use the 12-60 (which I don't know how it performs underwater) and I won't be able to upgrade with a wide angle anytime soon (moneywise). On the other hand I guess it would be easier to freedive with it, it has better ergonomics (does it?) and might be more future proof.
I must precise I'm studying wildlife filmmaking so this is a long term investment for professional use.
As I'm going to shoot an expedition in the Maldives in 15 days I'd like to have a polyvalent rig so it would be ikelite option or nauticam + gopro hero 6 fixed above for UWA.
I'm quite lost right now and can't settle on an option 
Help me please 