If you do some hunting, you should find lots of clip-art. Clip-art will add easily into GIMP, unlike that failure of trying to paste a text symbol character directly.
There also are extensions that work just fine in GIMP.
If you are having problems with specific extensions, you need to tell which extension you are trying, and give a link to the extension, then tell us the exact error you get when you try to use it.
Just saying you “tried” unnamed extensions and they did not work does not give us any way to help you.
You also need to learn to “Think outside the box.”
The internet is littered with millions of great clip-art graphics. All you need to do is find some you like, say on a website, download the graphics (or, even, screenshoot them, but please try to avoid copyrighted material), save them in files. If you want to collect them all in one file for easy selection to import into GIMP, I suggest you either open a file in OpenOffice Draw (or LibreOffice Draw) and import them into there, or you could create a GIMP file and import them all in there and arrange them on multiple layers all in one file.
After that, you simply choose one, use a selection tool to select it, copy it, then paste it into your project.
If you do that, you will have a collection of absolutely awesome graphics, dingbats, symbols, and so on for your projects.
Another saying:
“Anything worth doing is worth doing well.”
and
“You cannot do anything well if you are not willing work at it.”