Is the attic one large space, no wall, say, splitting the room into 2 rooms?

The biggest issue I see is spread of the wall. It can work and I have seen it work in older barns for many years. I also have seen failures most likely due to the spreading issue. As Mike eluded to, have you considered a ridge beam and supporting post?

The collar beams are not ties and won't do a whole lot to keep the spread at bay.

I believe your rafter size is fine. and you just have to love metal roofing!

Now go over to the ff for more opinion. And while your over there tell the guy with the bodger type shaving horse to put a removable shim on the rest for adjustments, I think he is from Norway. Or the clamping bar can be made rectangular, rotating it can give some adjustment as well.