Is there a way to keep the background asteroid layer on top of the planet so it's seen as between the planet and the platform, or is it a bad idea because those sprites have transparency?

Currently those asteroids float behind the planet which doesn't make sense to my eyes.

As far as I know, it's not possible as they're a part of the background tiles. Even if I could render behind tiles, it would also make the planet render behind space itself.

(Also this is already listed as a known issue on the mod page, just fyi)

True, it's listed as something barely noticeable though; for me it was one of the first things I noticed (something a bit jarring.) But I get that it probably won't be fixed.

Honestly I'm silently hoping that Factorio 2.1 will implement this mod into the base game, which will hopefully fix a lot of the issues that this mod is stuck with and look nicer overall :)