Given the circumstances of this, if i was using a scupper cart, instead of replacing a cart, I would be replacing the hull of my kayak.
Agreed again with most of your post OWF with the exception of the quote above.
For the last 5+ years I have been using a scupper cart in my twice weekly commute (on hard surfaces) using the AI hull only. I wheel the kayak about 400km/year and have had no scupper problems at all. The welds have broken on the cart (I have strengthened them with added reinforcing) and I have worn out 8 sets of wheels but the scuppers remain strong. Adding tennis balls as suspension on the cart posts has helped a lot.
The new Wheeleez tough wheels are amazing. After about 800km they still have plenty of tread and flats are a thing of the past.
The C-Tug cart excels in being really cleverly designed, easy to stow, light and strong. I have never really liked the large flat pads which can cause hull deformation, though that has stopped now I'm using Hobies TI cradle support. The greatest difficulty with a scupper cart is getting it back in when retrieving the kayak. The C-Tug on the other hand makes retrieving a simple job. It is so easy to slide it in place. I will try reversing the feeding of the strap through the buckle, though I only had a problem with the strap undoing when the cart got stuck in sand.
The Sidewinder measures 6" across which looks to be the maximum width wheel the C-Tug cart could take. It would be great if C-Tug could develop a proper sand wheel, maybe in conjunction with Wheeleez the experts in sand tyres. All that really needs to be developed is a wheel that fits the shorter C-Tug axle, yet can take a 6" wide balloon tyre. The latest balloon tyres are much more puncture resistant. I have had no punctures with the Trax2 tyres.