Is it possible to FULLY restore a Wii after homebrew?

In theory, it is possible to remove all traces of Homebrew from a Wii. There are tools that can generate a fresh NAND backup from the Nintendo server (for example, https://gbatemp.net/threads/ohneswanzenegger-wii-entry-pc-software.279750/ ), and you could theoretically flash a whole fresh NAND onto a Wii.

However, that is not completely risk-free, if the Wii crashes or loses power during the time it takes to restore the backup it's permanently bricked and can be thrown away.

Plus, in my opinion there is never a need to completely get rid of all homebrew traces. There should be no Wii anymore that still has Nintendo warranty, and the few traces that are left when you just uninstall all cIOS (using a WAD manager) and the Homebrew Channel (either using the Wii menu itself, or the hackmii installer) are negligible and don't cause trouble.