> Given an existing collection - Is there an easy way to auto sort & tag everything? e.g. Merge the artists 'Guns N Roses' and 'Guns and Roses' into the most correct one.

I've recently started using Beets[1] to organize my music collection. It's a command line application that IMHO is not entirely intuitive to use at first. But once you get the hang of it, it works incredibly well.

[1]: https://beets.io/