Looks great so far!

It'd be unwise to remove the (non-scale) balsa diagonals without replacing them with something else. It's not just a matter of "strength" but also lateral rigidity. That is, it's important that the tail can't twist. So just strengthening the longerons isn't enough.

One possibility would be to replace the single diagonals with crossed pieces of bamboo rod/skewers. Look for something about 1-2mm in diameter. Each individual piece doesn't have to be strong as it the net effect of having these cross-bracing pieces on every open frame (top, bottom, and sides). The thin bamboo will also look reasonably scale with backlite.

I've seen another builder here on RCU using kevlar threads to do the bracing and it looked very strong. I think I ended up using fine gauge music wire, which was probably not the ideal solution. But it looked good. I even wrapped bits of tape around the ends of each wire to give the effect of turnbuckles when seen in backlighting.