So, with everything (theoretically) fully completed up to this spot, I'm going to need to tear down a pretty long segment of the trunkway, and rebuild it to fit the new belt lines.
That means setting up a power bypass, for the 3 different power lines running inside the trunk (main power, miners for the Versatile Framework factory, and mines for the Ficsmas factory).
Also, need some scaffolding for placing all the belts and blueprints.
With that, we're clear to rip the trunk down to just the existing belts.
Rare opportunity as we're placing down new belts adjacent to existing belts, to compare curve mode to the old approximations we could do with separate straight mode segments. The difference is WAY more subtle than I expected. Curve mode looks great, but the older belts are close enough, I ended up being fine with just leaving them as-is.
And that's all the new belts in place.
A handful of blueprints later, and we're all rebuilt. At least, up to the point where we're going to have to add one more belt of resources, that we've not yet run. That'll be next time.