The reason you can't simply change the fill is because they are symbols. Changing the properties of a symbol won't change the entities it is made up from. (May feel weird for a simple symbol like this that is just a single entity, but symbols are often made up of many different entities with different purposes, so just changing them all doesn't make sense). Symbols needs to be exploded before you can change the properties.

The Dundjinni Archives also have some Sci-Fi stuff in them

On my phone right now, so I don't have a detailed guide for you but the easy way to approach this is to use a symbol scaled at 1.0 as a guide when making your new symbol.

When you define a new symbol, CC3 assumes the size you make it in is the default scale.

And scaling it to 1.06 in the symbol manager won't do much, as that just enlarges it by a tiny amount. To set it right you need to scale it to something like 0.22. Remember, scale 1.0 is the size you made it at, so anything above 1 when scaling makes it larger.

This happens because the background is a fill without scaling configured. This means it will repeat to fill the area depending on the resolution, so a high-resolution export will have many more pixels than on-screen display, thereby causing to repeat more.

To fix it, you'll need to click the fill style indicator in the top right, go to the bitmap files tab, find the relevant fill in the dropdown, and then check the "Scaled" option for the fill, and provide proper values for the X/Y size of the scaling. You'll need to experiment with those values to find the correct setting, but once you've found something that works on screen, since it now is scaled, it should then be the same size on exports.

You need to make three copies of the fill. Just select the fill you want to duplicate in the fill style dialog, and hit New and give it a name. This will give you two different fills, but still pointing to the same file on disk, and you can then adjust each individually. Just repeat for the third one.