And then there were three

 I spent a few hours today assembling the other 2 required diamonds. Here's the trio sitting where they will live. I have to paint them first, as well as add in all the regular ties required before gluing these guys down.

While fiddly, they weren't that hard to build. Just lots of little pieces of rail to create the guard rails and the inner bits of load bearing rails. The rails laid across the 75 degree diamonds are there to hold everything together until all is secured in place.
Wiring these would be a challenge but Hex frog Juicers can be used to help control polarity through the various frogs. Not that I need to do that. The L&PS run may not get wired. Depends how brave I'm feeling.