I have pulled my loaded Oasis on scupper cart by bicycle from campsite to ramp (about a mile) and would not want to go any further or do it often. I also have considered pulling with an eBike.
But for regular transport of 5 miles I would want something where the TI is carried and not a load bearing component in the transport system. That means not using the Mirage Drive mechanisms, the scuppers, or the bow handle eye. The bicycle trailers out there are mostly too narrow, too short, or have too low a load capacity, as I'm sure you found.
Don't know if you are a DIY person (you did mount a caster in a mirage cassette!) but these guys have a clever design that could serve as a starting point. Though, as they note, PVC probably wasn't the best choice for the spine.
http://glench.com/make/kayak-trailer/If I were building a bicycle trailer I'd use the 16" Skyway Tuffwheel (
https://www.bikesatwork.com/store/product/skyway-tuffwheel-16) rather than spoked wheels. They also have trailer axles but the widest at 32" was too narrow for my Oasis let alone a TI.
Peter