Joined late so just my summary thoughts.
One of my early machines was 600 wide and used a single screw and you could push each end of the gantry back and forth noticeably. I tended to cut in the centre line to hold a tolerance. So anything over 600 would be a double screw for me.

I used T5 belts on an earlier machine but use exclusively HTD 5 now. I prefer the tooth profile on HTD and believe the shape is optimised for high torque.
It has been a while since I looked into it but it think the AT types are somewhere similar in profile but I find the HTD work well.

Although I’ve only ever used rolled screws I have never found them to be anywhere close to being a limiting factor on the overall machine. So in my view rolled threads can be used and still make a very good machine.