Yes, no need for texture if you print evrything.
But why bother faking it and printing when you can just use a brush and paint it?
Anyway, I never print, I live in the digital world 
I only stack everything in a hard drive and forget about it. Printing will only fill the house…
Oh btw the canvas texture is optional.
Here is a bit of sky restoration from the @Ofnuts original, only built-in GIMP fx. I’ve lost track of how many cooks were in the kitchen. But this is a good “thinking exercise”.
Have you tried converting the image to indexed mode? I played around with this several years ago, and cannot fully remember all the details. In essence, change the image to index mode, and select how many colours you want - so far like posterise. But, if I remember rightly, you can then save the colours used to a palette. You can then edit that palette to change the colours and reapply the palette back to the original. To further process the image, you have to change the image back to RGB mode. Sorry, if this sounds a bit hazy, but it is at least ten years since I last tried.
Writing this reminded me of the technique and I have just been playing around. I am currently doing a project photographing the vegetation on verges, margins and waste land, to highlight the biodiversity of such places. This is of course green on green. I have just done a quick test using indexed mode, and found it quite helpful separating out very similar colours.
Sorry, I see you already have tried. Apologies for not reading the topic more fully,
No sweat. Both Indexed and Colors>Posterize have a similar ability. I like Posterize better for two reasons: 1-It shows me a preview the instant I select a new number of colors, and 2-The aesthetics are more pleasing. I pulled up a test image just now and tried both tools. I noticed that, with Indexed, there was some odd look like gravel in an area that was supposed to be smooth.
This might be color dithering which, as far as i know, simulates smooth transitions with noise.
Well, yes, but actually no.
It looks like noise, but noise are attributed to randomly generated values, and dithering usually doesn’t involve randomly generated values, but ruleset to defines how colors are inserted.
@Reptorian Damn, my first answer was “simulates smooth transitions with dots”. Changed it at the last second.
I briefly remembered that, but what I was seeing was more like golf balls than dots. This is a 300ppi image tested at 16 colors. Here is the section I noticed. From an image of small trees in a field, in fog, sidelit by morning.
Really looks like dithering.
You can deactivate this in Gimp when you index the image to 16 colors:
Here’s the difference :
Dithering:
No dithering:
Wow; two things going on here …
I almost never touch Indexing and don’t know much about it.
Also, for some reason, I thought dithering was preferred. Turns out, not always!
You did vegetation on your filter?
What a silly question! Of course I did, I don’t like empty spaces.
That looks like a dairy farm. First you’re supposed to pasteurize it.
