Have you ever used rub n buff? would that be a viable option?
And where you would use bondo I use apoxie sculpt
it gives me about 2 hours worktime and I can get it pretty smooth with my fingers by keeping my fingers wet with water or alcohol and rubbing it. I also can do a lot of the detailing in the apoxie sculpt that bondo won't allow due to dry time. I can build up ridged, tunnel channels, and it sands just like bondo...
the pitch-->(Apoxie® Sculpt offers economy as well as performance. This 2-part product has a putty-like, smooth consistency, and is easy to mix & use. It is safe and waterproof with 0% shrinkage/cracking! Working time is 2-3 hrs. Cures hard in 24 hrs., and has a semi-gloss finish. Adheres to plastic, resin kits, wood, metal, ceramic, glass, polymers, foam, fiberglass, & more! Can be seamlessly feathered before set-up, or sanded, tapped, drilled, carved, lathed, or otherwise tooled after set-up, without chipping, cracking or flaking!)
but you are doing some great work here... your attention to each piece is pretty astounding. I think the final will be amazing.