Well, filler/primer must be sanded, like it or not. Of course, there is the option to not use filler/primer, but it really does work nicely. On some non-critical future build I'll probably try swapping it for regular primer, and see how it goes.You are far more dedicated to sounding down primer coats than I am.
As a beginner BAR I used Chris Michielssen's prep techniques as a starting point, most of which were completely foreign to me. This includes, among other things, shooting the components with filler/primer before assembly. That works great and it is far easier to sand pieces of balsa before assembly *but* it starts to get unwieldy when there a lot of small parts to corral. So I switched, for now, to assembling first and then shooting filler/primer on the whole model (first tried, with good success, on the APRO Lander II). Exceptions still apply, such as the helical pieces and rings in this build; sanding after assembly would have been very difficult. Unfortunately, in this case, the nose assembly is exactly the sort of nooks and crannies festival that is hard to sand after assembly. But it also would have been impractical to prime those pieces before hand. In hindsight, I possibly could have built everything but the strakes, then primed and sanded, and then attached the strakes. That would have made sanding super easy.
Every build seems to have a different set of judgment calls to make in this area. I'm still not really locked into a technique that I can apply consistently.