16-35 certainly looks like a good lens. I've been beating my head against the wall trying to figure out what to do about the UWA side of things, but a part of me of saying sell the Loxia 21 and grab the GM to cover everything from 16-24mm. I liked the 12-24, but 12mm is such a limited focal length for me. The times I did shoot at that focal length, I screwed up the framing so much that I had to crop back to around to probably 18mm FOV in post to get a decent framing lol.
Carlo, that's the good and bad about having all these options. It's hard to decide!
The 12-24 is a great lens in its own right.