Well, yes and no. You cannot tow a high capacity ready-to-float roro down the highway, but you can tow the pieces to where you can assemble before launch. Then it’s less interference from locals to just drive your building materials onto the roro and motor out to a second flatdeck barge configured as a floating drydock. Build on the drydock, then lower the drydock, and float off the completed new modules. The seastead will want a floating drydock anyhow, for maintenance of the small stuff. It’s why i suggested using any public ramp as material access to land. Still need a SSB tho, for snail mail etc…
I don’t know where on the pie-shapes the 5x5x5m refers to, but i just priced 1/4in/6mm common steel plate for a 16x16 area in the $2,600 ballpark. If you multiply that by 3.5 to cover three 5x5m exteriors and a bottom, that’s $9,100. Gotchas, bracing, welding rods, paying me, etc, not included.