quick question, why are you trying to use DXT instead of just using a background image?
DXT is usually used for clouds/patterns stuff that you repeat as in webgl/opengl to create little things that look "big" they are not meant to replace the usage of jpeg,pngs textures.
while unity can compress the hell out of them, you can use png compression on your exported spritesheet once exported like tinypng etc... just make sure the size of the spritesheet doesn't change, you can bring down a 5mb to 100kb or less if done right.
also from what it sounds, loading the gpu memory by 200kb always doesn't seem a good approach, as images rendered in construct they render once, and then they just sit there they never get checked again by the gpu, unless you move them, that making it actually saving more memory than using, the only difference is with a preloaded texture image, you will have a loading wait for the 1st time on the start of game, other than that everything should be fast enough to be handled on a 2013 phone. anything older than that as a device (memory wise as an example take a A3 samsung galaxy 2013, if it runs on one of those will run on anything) is not worth optimizing for.
also so you can see what happens when you draw complex images in shaders take a look at shadertoy, my gpu cracks down every 2 demos i click (cause the gpu memory gets hogged down, and never clears unless im hard refreshing the cache there is tons of issues with that approach, since every pixel is drawn over and over again by your gpu shaders instead of just render in screen and check position/color change)... cause they are not meant to be used like that unless you run a workstation for that purpose.
having a 18mb 4k resolution background image sure is cool if you run it on a 4k monitor and im not talking about the 1920x1080 monitors that claim to be 4k... but an actual 4k monitor... that is 160cm wide or larger (also if you don't have a 4k gpu card, and a monitor/tv to be able to run 4k the detailing is pointless to have), if you do your game for majority of players a 1080p texture should be sharp enough to offer the quality and the detail needed. Mobile wise, you don't need textures greater than 480p.
in short if you want to use DXT1 = bmp images or flat lossless jpg
DXT3 accurate pixel drawing but no alpha good for patterns flat jpeg/bmp no transparency
DXT5 inaccurate pixeling for clouds and stuff that are blurry in background and don't require much sharpness = somehow a better compressed png.
difference in compression is about 50-1200kb depending on the compression you use on the png textures, memory usage while rendering images it will be insane huge as DXT always redraws the pixels, think of it as GPU idle usage around 10-20% still images and above if you move them (depending on device % - mobiles probably unplayable ).
and sure DXT is mostly used in 3d games, but if you are trying to do a 3d game in C3/C2 is probably not the best tool for that, you can do it, but you will probably have to import everything from a external source and just export with C3 but that will make C3 just a wrapper to export your game and not the engine you made the game in.
sorry if i sound a douche lol... i just don't get the convoluted way of doing a simple replica of the game you mentioned. it looks 2d to me, no need for 3d stuff, but just the illusion of 3d, the player won't care that much if your gameplay is great.