Personally I like tiled2002 (though you might have to select all files or put *.* in the name thing to force it to open whatever you like) and crystaltile2 as actual editors.
https://www.romhacking.net/utilities/818/ (click on the DS icon on the top right, it will open a window containing the files of the DS, and possibly indicate compression but it is spotty there. Double click on a file to be taken to that location in whatever of the windows you are in -- most likely the tile viewer or hex editor ones for this sort of thing but works for them all).
https://www.romhacking.net/utilities/112/ (bit more crashy and has some trouble with the huge sprites the DS sometimes uses but has some nice features as far as palettes, editing and reading savestates in GBA games so I keep it around)
For simple viewing then tiledggd is one of the more configurable tools I have ever seen. It lacks editing options but if you have to narrow down a conventional 2d tile format you don't know this will allow you to get there (most other tile editors instead offering a list of prebaked formats it knows about)
https://www.romhacking.net/utilities/646/
The utilities section of rhdn above is also host to many more, however most don't feature the GBA colour modes, and sometimes if they do it is only the 4bpp one and not the 8bpp one (never mind the more exotic ones seen on the GBA and DS that crystaltile2 supports).
tinke (
https://github.com/pleonex/tinke/releases ) is a good tool but at least was something of an example of the "do what we know" rather than "allow me to do what I want even if it is theoretically silly*". It does however do better with 3d textures (which many games use to varying degrees -- I often have to note for people that New Super Mario Brothers is technically a 3d game) than the others above as DS 3d textures do things even something as configurable as tiledggd is not built to handle.
*we have seen people edit hex in a tile editor, and more practically quickly scanning a visual representation of a file often tells me more about its contents than scanning through hex -- you yourself did a version of this when you said it was random junk. Certainly helps with eliminating those things that are simple patterns if I want something that will not appear as such.