@Soobel  
I dont have my test sheet with me and my calipers need new batteries, I want to say I was able to pull a line with my airbrush that was the same as a micron 02-05 which is .3 to .45 mm thick, but again, I am using paint that was never intended to be sprayed and I thinned it with a windex, water, and glycerin mixture with very cheap acrylic generic paint with proper I could likely pull a very small line with ease, though as small as you need? probably not. Personally I would sharpen a toothpick, spray on the pain I need pull the lines from the sprayed on paint, then if needed touch up
 
An airbrush may not be the be all end all for your painting, but from the look of celestia, getting 80-90% of the way would go far. Just looking at how thick I think the paint is on celestia, even if you had to go through and add small details with a brush I think an airbrush would greatly improve it, especially if the clay/whatever you used model underneath it is well done.
 
Not to mention that the larger things like the houses could also be done with the airbrush too, like I said, with mine I was able to put 2 coats of paint on a surface, get it to have complete coverage, and still be able to see the painted on decals shadow. I would at least look into it.
 
On a side note, there is an airbrush that would easily pull the lines you want, but 500$ is kind of a lot to tell someone to get.  
I would seriously consider getting a used one from someone who just wants to get rid of it and play around, see if you like it, harbor freight has some airbrushes for 20~$ that are fairly good as I doubt most people would have gotten a good airbrush to play around with when they had no idea if they liked it or not.

Hmm how thin are “very thin?” My ponies have eyelashes, what are thin like hair fibres and 0,3-0,5mm long, im not sure that they are possible to draw just with airbrush. Body and mane-tail will paint with airbrush, but i believe that eyes and CM must make with the edge of a “000” size usual brush.

@Soobel  
if you were inclined, you could look into a cheap airbrush  
https://www.amazon.com/Master-Airbrush-Multi-purpose-Dual-action-Compressor/dp/B001TO578Q  
That one there is specifically mine, with this I can pull very VERY thin lines, but because I am very cheap and I diluted acrylic paint/house paint, the stuff is a bit thick even when thinned so maintaining thin lines is not easy
 
I mostly say look into how they paint because it looks lumpy, but the the scale that you are doing this on is very hard to assess as my only real experience of doing something on a small scale is airbrushing, and with my diluted acrylic i’m able to lay down such a thin yet covering coat that if I am painting lets say some plastic with lettering painted on it, you can actually see where they lettering is when you shine light on it.
 
you also appear to have a chip on the wing. from what I know about mini painting, the thicker you put paint on the more likely it will chip.
 
The second thing, brushing the models down when they are still malleable is mostly because while some of these areas look like they were painted really thick, a number of the areas I can only imagine got the way they are because of the clay itself, and smoothing that out would lent itself to looking better and being somewhat easier to paint.
 
If you are going to make a crap ton of mini things or things that need to be painted one color in large areas, I would look into craig’s list and airbrushes, people always want to get good at airbrushing, but end up either giving up or stop trying, so you can get fairly good setups for dirt cheap.
 
I honestly don’t know what you can do with the paint or where the issues crop up, is it just clay, or is it paint and dust/particles…
 
But again, for its size its fantastic, I’m unsure if it could be better without lots of experience in painting minis or if your paints can be thinned well, all I really see is thinning them for airbrush use online and that’s far from ideal for brushing them on.

Both Revell’s and Humbrol’s white paints are specially very thick like toothpaste, i thin them with universal “white spirit” brush washer dilulant, but that thinner vaporize very quickly away.

@alidan  
THIN YOUR PAINTS!

if you are interested in help, look up some how to paint warhammer models, they are very helpful with getting the best paint job possible, and not sure what you used to model them with, but if it was clay, look into if you can brush them down with rubbing alcohol, some clays you can, it picks up a small amount of the clay, and dries up almost instantly making is a good method of smoothing out small models.
 
other then that, they look fantastic for how small they are.