1. I said the real issue was the front because I don't think the faults with the back would be nearly as noticeable if not for the front half being so much fatter (which draws attention to the smoothness there) than it because of the train mode kibble. But thinking about it, that was still wrong. It's not the real, real issue, it's what rips the wallpaper off the main issue (which I'll get to).
2. I reiterate my opinion that the tender is an entirely necessary thing that the 1986 Astrotrain toy was incomplete for not having, and I think the crawler/launch base is a better shuttle-mode use than covering the rear of the shuttle (which they
did put effort into, as shown by details like the vents).
3. The tender has nothing to do with the back end's appearance flaws anyway. His back end would look just as flawed, maybe even worse, if he'd been made as a tenderless Voyager. Why? There are two factors in the appearance of the back: One is the fact that Astrotrain's arms have to tuck in there, and they're bulkier relative to the rest of him than those of the 1986 toy. The other one, and the one more damaging to the looks,
is their insistence on giving Astrotrain a cartoon-accurate (in his case, wrong) chest.
See, part of Astrotrain's original design is that the shuttle tailfin is
supposed to slide down and split open to give him a chesplate:

But the animation model neglected that step, and was drawn based on his toy with the fin tucked in, and stylized it beyond that, so it looks like this:

Since they decided to go with the cartoon chest, and the back of the shuttle becomes his torso in robot mode, it ends up taking a hit in the looks department to accommodate that chest (because of how it has to fold up).
The Titans Return version did accommodate the cartoon chest more cleanly, but A. that one cheats by moving the wings to the top of the shuttle mode and B. That one also doesn't bother tucking the arms away.
If they'd kept the chest more faithful to the original
toy (as the Classics figure did), I'm fairly sure the rear of the shuttle would have been smoother up top.
So IMO the kit we need is not some partsforming shield/cover, but a replacement for the stupid smegging cartoon chest. My desire for cartoon accuracy does
not extend to dumbed-down details or to animation model snafus. Deliberate, thought-out changes like color differences or Bumblebee having a face? Yes. Snafus like Astrotrain's chest or everything about the Combaticons but their heads? Not so much.