The power of a big scope is in the delivery of photographic images of deep sky objects. To see those views, though, requires dark skies.
If you're stuck in brighter skies, and you primarily look at the planets and the Moon, I see no reason to go larger than 10-12" because it is HIGHLY unlikely the seeing
will ever exceed the resolution of that aperture. The really big dob is simply wasted in that environment.
The 60" at Mt. Wilson, for example, doesn't give any better image of DSOs than a really good 16-18" in dark skies.
So back to collimation, which affects high power images most: the ultralight won't hold collimation tolerances you need for high powers on a fast f/ratio.
Yes, it's possible to engineer and make your own ultralight and stiff scope. But you aren't describing any commercial scope I know of.
So figure you're going to build it yourself.
There is a way around it, I suppose: get a commercial ultralight and start modifying it with larger diameter poles or braces between the poles, or guy-wires to stiffen the structure,
and perhaps mods to the cell and trunnions. That might yield a stiff-enough scope without engineering the entire scope yourself.
I would add more carbon truss rods to stiffen up an ultra light if required. I have been tempted by the Obsession UC. Almost bought an 18". but we could not agree on a price.