Any links to a completely safe guide for this, for safely uninstalling Unlaunch on an EU DSi in particular?
You are not going to find what you seek. Every possible method carries
some inherent risk.
Bricks can happen when uninstalling (ignore the two replies, OP clearly states that they used the Uninstall function in the menu.) The uninstall function is only safe if you
know that your nand is completely clean. And unlike the 3DS, your only way of unbricking if uninstalling mods bricks you, is via a hardmod, which carries an inherent risk of ruining your motherboard if you make a mistake.
Unless trading in a modded console would get you into trouble with the law in your country, I really wouldn't recommend uninstalling. Alternatively, you could see how much your local store is willing to buy a secondhand DSi for, and lower your sell price on your local buy and sell website accordingly so you can sell it there instead.